The maneuverable reentry vehicle (abbreviated MARV or MaRV) is a type of warhead for ballistic missiles that is capable of maneuvering and changing its trajectory.
There are two general reasons to use MARV. One is to make it more difficult to track the re-entry vehicle (RV) and thereby make it more difficult to attack as it approaches its target. This was particularly useful against early anti-ballistic missile (ABM) systems which took seconds to calculate an interception course. Making random trajectory changes could render these systems useless. This class of MARV is sometimes known as evading MaRVs.
The other is to improve accuracy or track moving targets using terminal guidance systems that can act only during the last stages of the flight. This class is sometimes known as accuracy MaRVs. In this case, it is the short range of the active guidance system that demands the RV be able to maneuver, as is the base in the Pershing II active radar homing system. The same systems may also be used to track moving targets like aircraft carriers, which move far enough between launch and approach that there is no way to predict their location and active terminal guidance must be used.