Card Sorting
Letter CMethod for observing how people group and label content. It yields evidence about mental models; it does not define the architecture on its own.
Updated:
Working definition
Card sorting asks people to group — and, in open versions, name — content items. It shows how they conceptualize a domain.
It yields evidence about mental models. It does not vote in a final architecture: that is decided with content, context, and later validation.
When to use it
When it is unclear what belongs together and what to call it, before drawing menus or sitemaps as if they were the decision.