being a single unit or thing. being one in particular. being preeminently what is indicated. being the same in kind or quality. constituting a unified entity of two or more components. being in agreement or union. SOME sense 1. being a certain individual specified by name. ONLY sense 2a. the first whole number above zero. the number denoting unity. the first in a set or series —often used with an attributive noun. an article of clothing of a size designated one. a single person or thing. a one-dollar bill. a certain indefinitely indicated person or thing. an individual of a vaguely indicated group: anyone at all. used as a third person substitute for a first person pronoun. a single instance of a specified action. ketone or related or analogous compound or class of compounds.