Plain English Meaning
Working out where to start coming down so you arrive at the next altitude restriction or the airport at the right height and speed, instead of being too high or too low.
Full Definition
The process of calculating when and how to begin a descent from cruise altitude so the aircraft arrives at a required altitude over a specific point at the right speed, using known groundspeed, desired rate of descent, and the altitude to lose.
Context Anchor
Seen during planning, , and any time a must descend to meet a charted or assigned altitude.