A cowling is a removable cover/panel on a vehicle or aircraft.
Cowlings are, for the most part, the outer surface of a vehicle. While the cowlings on human military vehicles are typically dark green, Covenant-made cowlings are usually purple or blue in color, such as those of the Wraith and Ghost. The Banshee is a good example of a vehicle with cowlings; they are purple in color and act as an outer 'shell' for the vehicle. When the Heretics rebelled against the Covenant, they removed the cowlings from their Banshees to reveal the mechanisms beneath, creating the unique appearance of their Type-26 aircraft.
All vehicles featured in the Halo universe feature some sort of cowling. They can typically be removed by explosions, melee, or other arms fire.