Mobile Construction Vehicle.
An MCV is the most important vehicle on the battlefield. It deploys into a Construction Yard, which is used to build buildings.
On the modding side, without the RockPatch, Red Alert 2 can handle a total of 2 unique MCVs, held within the BaseUnit list, whereas Yuri's Revenge can handle 3. With the RockPatch installed into Yuri's Revenge, the game can handle many more, allowing for unique MCVs per country, or for a new side.