Time step
The small slice of time a simulation moves forward in each update.
See it in Strange attractors. Raise dt and watch the curve turn jagged; lower it and the curve smooths out.
In plain words
A simulation cannot move time forward smoothly; it jumps in small steps. Each step, it works out every speed and position for a tiny slice of time. That slice is the time step, often written dt. Small steps are accurate but slow. Large steps are fast, but paths turn jagged and a simulation can even blow apart.