Plain English Meaning
Extra miles added to where you start your descent because a headwind is slowing you down. The stronger the headwind, the sooner you have to begin coming down to reach your target altitude on time.
Full Definition
A correction added to a planned descent distance to account for the slowing effect of a headwind on groundspeed during descent. Because a headwind reduces the distance covered per minute, the descent must be started earlier (farther from the destination or fix) than the still-air calculation would suggest.
Context Anchor
Seen when planning the top of descent or checking whether a high-performance aircraft can meet an altitude restriction during descent.
Derivation
“Headwind” means wind coming from ahead of the aircraft, against its path of travel. “Adjustment” means a correction or change. Together, the term points to correcting the descent plan for the from ahead.