An excise is a tax upon an event or privilege. A motor vehicle and trailer excise is in lieu of a tangible personal property tax and is levied for the privilege of registration.
M.G.L. Ch. 60A, which is the statutory basis for the excise, uses the motor vehicle itself as a means to measure this privilege. Revenue derived from the excise can be used by cities and towns for any lawful purpose.
For more detailed information regarding the definition of a "motor vehicle", the period of assessment, and certain registration requirements, see: General Motor Vehicle Excise Information (PDF).