- 1[uncountable] the most important or best person or thing We're number one in the used car business.
- 2[uncountable, countable] the song or album (= collection of songs) that has sold the most copies in a particular week She's had three number ones. The new album went straight to number one.
- 3 [uncountable] yourself Looking out for number one is all she cares about.
- 4[singular] an expression used especially by children or when speaking to children to talk about passing liquid waste from the body It's only a number one. compare number two