noun: (countable, sometimes capitalized) A publication containing biographies of well-known or important people.
noun: (countable) A list of notable, famous, or upper-class people.
noun: (uncountable) Collectively, the elite class.
noun: (countable) A person appearing in a Who's Who publication.
noun: (countable) A celebrity or famous person, someone likely to be in such a publication.
noun: (uncountable, idiomatic) The identities of specific people, understood in terms of such distinguishing characteristics as their backgrounds, prominence, achievements, jobs, etc., as a basis for comparing them and especially as a basis for ranking them within a social group.
noun: Alternative form of who's who [(countable, sometimes capitalized) A publication containing biographies of well-known or important people.]