To increase the size, volume, quantity, or scope of; enlarge: expanded her store by adding a second room. See Synonyms at increase. To express at length or in detail; enlarge on: expanded his remarks afterward. To open (something) up or out; spread out: The bird expanded its wings and flew off. Mathematics To write (a quantity) as a sum of terms in an extended form. To become greater in size, volume, quantity, or scope: Air expands when heated. This critic's influence is expanding. To speak or write at length or in detail: expand on a favorite topic. To open up or out; unfold: The chair expands to form a day bed. To feel expansive.