What is Delta Time

Is Delta Time the time period between the beginning of this tick and the beginning of previous tick, or is it (and how come?) the length of the current active tick (which hasn’t ended yet).
This might seem trivial if DeltaTime was always similar between consecutive frames, but sometimes, the difference in DeltaTime is really big from frame to the next. In this case the exact meaning of DeltaTime is critical.

IIRC it is the time since the last tick.