To box out is to position your body between an opponent and the basket after a shot goes up, blocking their path to the rebound.
The box out is rebounding's unglamorous foundation. When a shot goes up, the defender turns, finds an opponent, makes legal contact with their body, and holds position between that player and the basket, so the rebound can be collected by themselves or a teammate. The box score credits the rebound to whoever grabs the ball; the box out that created it appears nowhere in the statistics.
Coaches measure teams by it anyway. Defensive rebounding percentages collapse when players watch shots instead of boxing out, and offensive rebounders build careers defeating the box out with timing and angle. The phrase has escaped into general use as any act of positioning a rival out of contention, which is close to its literal meaning on the glass.
Historically, the great rebounding teams were great boxing out teams, and the skill mattered more as size differences shrank: a smaller player with perfect box out technique can neutralize a taller one for a whole series. Quiz questions about rebounding records are often secretly box out questions, since the totals belong to players whose positioning made the ball arrive where they already were.
In use"She boxed out the taller centre and let the rebound fall straight to a teammate."