Search
Letter SSystem for retrieving information when someone already has a query, or when navigation is not enough.
Updated:
Working definition
Search retrieves information from a query. It complements navigation: one assumes the person already knows what to ask for; the other assumes they can walk a structure.
In IA search is not a magic field. It depends on how what exists is named, classified, and related, and on the search experience (results, empty states, facets).
Criterion
If people search for what the menu cannot surface, there is an access problem. If they search and do not recognize the result, there is a labeling or model problem.