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CDL Glossary — 63+ Terms & Definitions

A complete reference of Commercial Driver's License terms, abbreviations, and definitions. Essential study material for your CDL permit test. Last updated March 2026.

Licenses & Permits

CDL
Commercial Driver's License. A license required to operate large, heavy, or hazardous-material vehicles in the United States. There are three classes: A, B, and C.
CLP
Commercial Learner's Permit. A temporary permit that allows you to practice driving commercial vehicles under the supervision of a CDL holder. Valid for 180 days in most states.
Class A CDL
Authorizes driving combination vehicles with a GCWR of 26,001 lbs or more, provided the towed vehicle weighs more than 10,000 lbs. Examples: tractor-trailers, truck and trailer combinations, tanker vehicles.
Class B CDL
Authorizes driving single vehicles with a GVWR of 26,001 lbs or more, or such a vehicle towing another vehicle not exceeding 10,000 lbs GVWR. Examples: straight trucks, large buses, dump trucks.
Class C CDL
Required for vehicles designed to transport 16 or more passengers (including the driver) or vehicles carrying hazardous materials requiring placards, when neither Class A nor Class B requirements apply.
Endorsement
An additional authorization added to a CDL that permits the holder to drive specific types of vehicles or carry certain cargo. Common endorsements include HazMat (H), Tanker (N), Passenger (P), School Bus (S), and Doubles/Triples (T).
Restriction
A limitation placed on a CDL that prevents the holder from operating certain vehicle types. Common restrictions include air brake restriction (L), manual transmission restriction (E), and intrastate only (K).
ELDT
Entry-Level Driver Training. Federal requirement effective February 7, 2022, requiring new CDL applicants to complete training from a registered provider listed on the FMCSA Training Provider Registry before taking the CDL skills test.

Weight & Measurement

GVWR
Gross Vehicle Weight Rating. The maximum allowable total weight of a single vehicle including its cargo, fuel, passengers, and equipment as determined by the manufacturer.
GCWR
Gross Combination Weight Rating. The maximum allowable total weight of a combination of vehicles (power unit plus towed unit(s)) including all cargo, fuel, passengers, and equipment.
GVW
Gross Vehicle Weight. The actual total weight of a single vehicle plus its load at any given time, as opposed to GVWR which is the maximum rating.
Curb Weight
The weight of a vehicle with standard equipment, all necessary fluids (fuel, oil, coolant), but without passengers or cargo.
Axle Weight
The total weight transmitted to the road by all wheels on a single axle or tandem axle group. Federal bridge law limits single axle weight to 20,000 lbs and tandem axles to 34,000 lbs.

Regulatory Bodies

FMCSA
Federal Motor Carrier Safety Administration. The federal agency within the U.S. DOT that regulates the trucking and bus industries, sets CDL standards, and enforces safety regulations.
DOT
Department of Transportation. The federal cabinet department overseeing all transportation systems in the United States. State DOTs typically administer CDL testing and licensing.
DMV
Department of Motor Vehicles. State agency responsible for vehicle registration and driver licensing. The name varies by state (BMV, MVA, DLD, SOS, etc.).
NHTSA
National Highway Traffic Safety Administration. Federal agency focused on road safety, vehicle safety standards, and crash data. Works alongside FMCSA on commercial vehicle safety.
CSA
Compliance, Safety, Accountability. An FMCSA initiative that uses data from roadside inspections, crash reports, and investigations to identify high-risk carriers and drivers.

Vehicle Systems

Air Brakes
A braking system that uses compressed air to apply force to brake drums or discs. Required knowledge for Class A and Class B CDL. Failure to pass the air brake knowledge test results in an L restriction.
S-cam Brakes
The most common type of foundation brake on commercial vehicles. When air pressure pushes the brake chamber pushrod, it turns a cam shaped like the letter S, which forces brake shoes against the drum.
Slack Adjuster
A lever arm that connects the pushrod from the brake chamber to the S-cam. Adjusts automatically or manually to compensate for brake shoe wear and maintain proper brake stroke.
Brake Fade
Loss of braking power caused by excessive heat buildup in the brakes, usually from prolonged braking on long downgrades. Prevented by using proper gear selection and the stab braking technique.
Spring Brakes
Parking brakes on air brake vehicles that use powerful springs held back by air pressure during normal driving. When air pressure drops below 20-45 psi, springs apply the brakes automatically.
Governor
An air compressor control that manages air pressure in the brake system. Typically cuts in (starts pumping) at around 100 psi and cuts out (stops pumping) at around 125 psi.
Glad Hands
Coupling devices that connect the air lines between the tractor and trailer. Two types: service (blue) for service brakes and emergency (red) for emergency/supply air.
Fifth Wheel
The coupling device mounted on the rear of a tractor that connects to the trailer kingpin. It is the pivot point between tractor and trailer, allowing the trailer to articulate during turns.
Kingpin
A steel pin on the underside of the trailer nose that locks into the fifth wheel to couple the trailer to the tractor. Must be inspected regularly for wear and damage.
Landing Gear
Retractable legs under the front of a trailer that support it when disconnected from the tractor. Operated by a crank handle; must be fully raised before driving.
Tandem Axle
A pair of axles grouped together on a vehicle, common on tractor drive axles and trailer axles. Distributes weight more evenly and provides better traction.
Converter Dolly
A coupling device with one or two axles used to connect a semi-trailer to the rear of a tractor-trailer combination, creating doubles or triples configurations.
Bobtailing
Driving a tractor without a trailer attached. Bobtailing increases stopping distance because the drive axles have reduced traction without trailer weight.
Deadheading
Driving a commercial vehicle without any cargo. An empty trailer can be more difficult to control in high winds and has different braking characteristics than a loaded one.

Endorsements

H Endorsement (HazMat)
Hazardous Materials endorsement. Required to transport hazardous materials requiring placards. Requires passing a knowledge test and a TSA background check. Must be renewed every 5 years.
N Endorsement (Tanker)
Tank Vehicle endorsement. Required to drive any vehicle designed to transport liquid or gaseous materials in a permanently attached tank rated at 119 gallons or more.
P Endorsement (Passenger)
Passenger endorsement. Required to drive vehicles designed to carry 16 or more passengers including the driver. Requires both a knowledge test and a skills test.
S Endorsement (School Bus)
School Bus endorsement. Required to drive a school bus. Requires passing a knowledge test, skills test, and typically a background check. Must also hold the P endorsement.
T Endorsement (Doubles/Triples)
Doubles/Triples endorsement. Required to pull double or triple trailers. Only available with a Class A CDL. Requires passing a knowledge test.
X Endorsement
A combination of HazMat (H) and Tank (N) endorsements. Required when transporting hazardous materials in a tank vehicle. Requires passing both the HazMat and Tanker knowledge tests.

Safety & Inspection

Pre-Trip Inspection
A systematic check of the vehicle performed before every trip to identify safety defects. Required by FMCSA regulations and is a graded component of the CDL skills test.
DVIR
Driver Vehicle Inspection Report. A written report completed by the driver at the end of each day documenting the condition of the vehicle and any defects found.
OOS
Out of Service. An order issued during a roadside inspection that prohibits a vehicle or driver from continuing to operate until identified safety violations are corrected.
Placards
Diamond-shaped signs displayed on all four sides of a vehicle transporting hazardous materials. They indicate the hazard class (flammable, corrosive, explosive, etc.) and are required when hauling certain quantities.
Shipping Papers
Documents that describe the hazardous materials being transported, including proper shipping name, hazard class, UN/NA identification number, and quantity. Must be within reach of the driver at all times.
Bill of Lading
A legal document issued by a carrier detailing the type, quantity, and destination of the goods being carried. Also serves as a receipt and contract of carriage.

Driving Techniques

Stab Braking
An emergency braking technique for vehicles without ABS. Apply brakes hard until wheels lock, release to let them roll, then reapply. Maintains steering control during hard stops.
Controlled Braking
Applying the brakes as hard as possible without locking the wheels. Allows the driver to maintain steering control. Preferred over stab braking when equipped with ABS.
Counter-Steering
Turning the steering wheel in the opposite direction of a skid to regain control. If the rear of the vehicle slides to the right, steer right to correct.
Jackknife
A dangerous situation where a tractor-trailer folds at the pivot point (fifth wheel), causing the trailer to swing toward the cab. Usually caused by braking on slippery surfaces or excessive speed.
Off-Tracking
The tendency of a trailer's rear wheels to follow a shorter path than the tractor's front wheels during turns. Requires making wider turns to prevent running over curbs or hitting objects.
Following Distance
The space between your vehicle and the one ahead. For CMVs under 40 mph, maintain at least 1 second per 10 feet of vehicle length. Above 40 mph, add 1 additional second.
Escape Ramp
A safety feature on steep downgrades, also called a runaway truck ramp. Uses sand, gravel, or an uphill grade to stop vehicles that have lost braking ability.

Hours of Service

HOS
Hours of Service. Federal regulations that limit how many hours a commercial driver can be on duty and driving. Designed to prevent fatigue-related crashes.
ELD
Electronic Logging Device. A device connected to the vehicle engine that automatically records driving time and HOS data. Required by the FMCSA ELD mandate for most CMV drivers.
11-Hour Driving Limit
Property-carrying drivers may drive a maximum of 11 hours after 10 consecutive hours off duty. This is the core HOS driving limit for most truck drivers.
14-Hour On-Duty Limit
Also called the 14-hour window. Property-carrying drivers may not drive after the 14th consecutive hour after coming on duty, regardless of breaks taken during that period.
30-Minute Break
Property-carrying drivers must take a 30-minute break after 8 cumulative hours of driving without at least a 30-minute interruption. The break can be on-duty not driving or off-duty.
60/70-Hour Limit
Property-carrying drivers may not drive after being on duty 60 hours in 7 consecutive days or 70 hours in 8 consecutive days. The cycle resets after 34 consecutive hours off duty.

Medical & Testing

DOT Physical
A medical examination required for all CMV drivers, conducted by a medical examiner listed on the FMCSA National Registry. The medical certificate is valid for up to 24 months.
Medical Examiner's Certificate
A card issued after passing the DOT physical that certifies a driver is physically qualified to operate a CMV. Must be carried while driving and filed with the state licensing agency.
Skills Test
The practical driving portion of the CDL exam. Consists of three parts: vehicle inspection, basic vehicle control (backing maneuvers), and an on-road driving test.
Knowledge Test
The written portion of the CDL exam, consisting of multiple-choice questions. Typically 50 questions for General Knowledge with an 80% passing score required.
TSA Background Check
A security threat assessment required for the HazMat endorsement. Conducted by the Transportation Security Administration. Involves fingerprinting and a criminal history check.
BAC Limit
Blood Alcohol Concentration limit. For CDL holders operating a CMV, the legal BAC limit is 0.04% — half the standard 0.08% limit for non-commercial drivers.

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