Delta time
The time elapsed since the previous frame, used as a multiplier so motion runs at the same speed regardless of frame rate.
An object moved with position += velocity every frame travels twice as fast on a machine drawing 120 frames per second as on one drawing 60. The fix is to scale by the timestep: position += velocity · Δt, where Δt is the seconds since the last frame. Distance per second becomes constant and the game is decoupled from the hardware.
The need became acute when DOS-era PC games that tied their logic to the CPU clock turned unplayably fast on newer machines — hence the turbo button that slowed the clock back down. Variable Δt has its own downside, though: physics becomes non-deterministic, and a single long frame (a hitch) can fling objects through walls. That is why many simulations run on a fixed timestep instead.