The MCV (Mobile Construction Vehicle) is a slow, deployable vehicle. Once deployed, it transforms into a non-moving Construction Yard. Under normal map options, the MCV is the only starting unit. The Allied MCV and the Soviet MCV look identical, but they transform into an Allied Construction Yard and a Soviet Construction Yard, respectively.
Since the MCV and the Construction Yard are highly related, sometimes their names are used interchangebly. More information about both can be found at the Construction Yard Article, which is the main article on the subject.