Delta time
Also said as dt, Frame-rate independence, Time step
The time the last frame took. Multiply every movement by it, and a character covers the same ground per second however fast the machine runs.
Without it, speed is tied to frame rate: the game plays in slow motion on a weak computer and at double speed on a fast screen. It is one multiplication, and old games that skipped it are often unplayable on new hardware.
Say it
“This speed is per frame, so it runs twice as fast on a 120 Hz screen. Make it 240 px per second and multiply by delta time.”
Where you’ve seen it
Dark Souls II tied weapon durability to the frame rate, so weapons wore out much faster at 60 frames a second on PC than at the consoles' 30. Patches in 2015 fixed it, about a year after players reported it.
Not to be confused with fixed timestep
Delta time feeds the real, varying length of each frame into your maths. A fixed timestep runs the maths in equal slices whatever the frames do.