Massive is the term for the ability of specific units to destroy other, smaller units simply by walking on top of them.
The Massive ability can cause the destruction of any land unit with the exclusion of Experimental units, ACUs, and SCUs, and is extremely useful as it effectively increases the possessing unit's destructive power. Additionally, this ability can allow the possessing unit to clear a path through obstacles, such as Walls, to enable smaller units or units without the Massive ability to pass through.
All land based experimental units possess this ability, excluding the Fatboy. See the massive category for a full list.