Initiative
The ability to make threats and set the agenda, forcing the opponent to react rather than create plans of their own.
The initiative is the abstract right to dictate what happens. When you hold the initiative, your moves create threats; the opponent’s moves defend against them. Their plans are postponed, their pieces are diverted, their time is consumed reacting to yours.
Initiative is the most valuable thing in chess after material itself, and sometimes more valuable than a pawn. Many gambits — the Smith-Morra, the King’s Gambit, the Evans Gambit — trade a pawn for the initiative on the theory that an opponent forced to defend for ten moves will eventually slip, and the pawn will return with interest. Whether the trade is sound depends on whether the initiative survives long enough to convert.
Compensation is the related concept that values material against initiative explicitly. A player who has sacrificed a piece for an attack has compensation for the material if the initiative is strong enough — open files, exposed king, active pieces. The compensation is real even when it is not yet material; the question is whether it converts before fading.
The initiative tends to be lost gradually. Defenders have a long list of small improvements they can make once the immediate threats run out. A player with the initiative must keep generating new threats; if even one move slows, the opponent consolidates and the initiative evaporates.