Line of sight (LOS)
The question of whether one point can "see" another with nothing in the way — the basis for enemies spotting the player.
Line of sight decides when a guard raises the alarm, what a unit may fire at, which tiles the stealth system counts as hidden and how fog of war lifts. In 3D it is solved with raycasting against the geometry; on grids with Bresenham's line, shadowcasting or a permissive field-of-view algorithm.
A classic grid pitfall is asymmetry: with a naive algorithm A can see B without B seeing A, which feels unfair. Shadowcasting and symmetric variants exist precisely to avoid that.