Search abandonment is the share of searches after which the visitor takes no meaningful action — no click, no refinement, no conversion — before leaving.
It is a stronger health signal than zero-result rate, because it also captures the more common failure where results were returned and none of them were right. A site can have a very low zero-result rate and still fail most searchers.
Abandonment is expensive precisely because searchers are high-intent. Someone who typed a query has already decided what they want; losing them is losing a visitor much further down the funnel than an average browser.
Measured properly it should be segmented by query, because the aggregate hides the pattern — abandonment usually concentrates in a small number of recurring query shapes the system handles badly.