Fixed timestep
Also said as Fixed update, Accumulator, Physics tick
Frames vary, so bank the time they take, and spend it on game logic in equal slices such as a 120th of a second. The picture can run at any rate while the rules always step the same.
Physics maths gives different answers for different slice lengths, so with a varying step the same jump reaches a different height on each machine. Equal slices also make a replay or a networked game come out exactly the same.
Say it
“Run physics on a fixed 120 Hz step with an accumulator, and draw at whatever rate the display runs. Cap it at five steps a frame so a long stall cannot snowball.”
Where you’ve seen it
Doom runs its game logic at a fixed 35 tics a second. Quake III Arena is the warning: certain frame rates gave slightly higher jumps, and its pmove_fixed setting chops movement into fixed steps, in code commented "to prevent framerate dependent behavior".
Not to be confused with delta time
Delta time accepts that every step differs and scales for it. A fixed step refuses to differ. Most engines use both: fixed for physics, delta for visuals.