Autocomplete is the suggestion list that appears as a visitor types, predicting queries, products or categories before the query is submitted.
Also known as: typeahead, search suggestions, instant search
It serves three jobs at once: it saves typing, it teaches the visitor what the site actually contains, and it steers queries toward phrasings the index can answer. The third is the most valuable and the least appreciated — a good suggestion list quietly prevents zero-result searches from ever being submitted.
The failure mode users complain about most is prefix-only matching: suggestions that appear only once a whole word has been typed, or that never match mid-word. Partial and fuzzy matching from the second or third character is the baseline expectation.
Suggestions should be mixed, not uniform. Showing a few products alongside query completions and category jumps lets a visitor skip the results page entirely, which is the fastest possible successful search.