Cover
Something between an attacker and the target, like a wall or bush, that makes the target harder to hit.
Cover is anything partly in the way of an attack: a low wall, a tree, another creature. Targets behind cover are harder to hit. Targets completely hidden behind something cannot be aimed at directly. The app works out cover from the map.
In Fantasy RPG
Fantasy RPG: partial cover gives +2 AC and cover gives +5 AC. A target completely out of sight cannot be aimed at.