From the Persian for "strike": the point where weight falls — the accent structure of a phrase rather than its raw count. Ustads of the older Urdu-speaking tradition use it for the felt attack of a bol or the stress a composition places inside the cycle.
Two renderings of the same bols can differ entirely in zarb: identical text, different placement of force. Listening for zarb — not just correctness of strokes — is how mature players evaluate a performance.