Raycast
Also said as Ray test, Line trace, Hitscan (for weapons)
Asking what a straight line from here in that direction hits first, and how far away it is. It is the most common question in a game engine.
Line of sight, instant-hit guns, checking for ground under your feet and clicking on things are all one question. It is cheap, so you can ask it hundreds of times a frame.
Say it
“For the guard's sight, cast one ray from his eyes to the player's head and one to their chest. If either arrives without hitting a wall, he can see them.”
Where you’ve seen it
Wolfenstein 3D draws its whole picture this way: one ray for each column of the screen, and the distance it returns sets how tall that strip of wall is.
Not to be confused with ray tracing
A raycast is a single query about the game world, used for gameplay. Ray tracing fires millions of rays to work out the colour of each pixel, used for lighting.