USDOT number
The federal ID number that identifies your trucking company to the FMCSA and follows your safety record.
A USDOT number is the identification number the federal government assigns to your trucking company. It is issued by the FMCSA and it is how they track your company: your safety record, your inspections, your crashes, and your compliance reviews all attach to that number.
Most carriers operating commercial vehicles in interstate commerce are required to have one, and it goes on the side of the truck. Getting one is free and done through the FMCSA. It is the first identifier your business gets, before you even start hauling.
A USDOT number is not the same as operating authority. The USDOT number identifies the company and carries the safety data, while the MC number is the authority that lets you haul regulated freight for hire. Many carriers need both.
Related: operating authority, IRP, owner-operator.