the best, most important, or most successful person or thing in a group:
Until his marriage, his job was number one in his life.
Shearson is number one in the market this year.
2APM [uncountable and countable]
the musical record that is the most popular at a particular time:
number one in the charts
They've had three number ones.
3 also look after number one spoken
to look after yourself and not worry about other people, in a way that may seem selfish
4 [singular, uncountable] spoken informal
a word meaning urine, used especially with children to avoid saying this