Faceted search (also called guided navigation) is the sidebar of checkboxes, sliders, and pills that turns a 5,000-result query into a 50-result browseable list. Each facet is a structured attribute: brand, size, color, price, average rating, in-stock status. The values shown are dynamic — they reflect what’s actually available in the current result set, with counts.
For ecommerce, facets carry most of the weight on broad category queries (“dresses”, “laptops”). The shopper rarely narrows by typing — they narrow by clicking. That makes facet quality (clean attribute data, no near-duplicates like “red” and “Red”) one of the highest-leverage data hygiene problems in a catalog.
Modern faceted search engines compute facet counts in the same query as the result set so counts always match reality. They also support hierarchical facets (Category → Subcategory → Brand) and multi-select within a facet (color: red OR blue) without recomputing from scratch.