A zero-result search is a query that returns nothing, and it is the single most damaging outcome in website search because the visitor reads it as "this site doesn't have it".
Also known as: null search, no results page
The causes are rarely absence. Typos, synonyms the index doesn't know, over-restrictive filters, natural-language phrasing, plural forms and non-product questions all produce empty result sets for content the site genuinely has.
A zero-result page should never be a dead end. Showing corrected-spelling results, relaxing the least important filter, offering the nearest category and giving a route to a human recovers a meaningful share of sessions that would otherwise bounce.
Zero-result queries are also the highest-value line in the query log: each one is a customer telling you, in their own words, about a gap in your catalogue, your vocabulary or your content.