I. ˌnumber ˈone 1 BrE AmE noun
1 . [uncountable] 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.
2 . [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 . look out for number one ( 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
II. ˌnumber ˈone 2 BrE AmE adjective
1 . most important or successful in a particular situation:
The University of Maine has the number one hockey team in the country.
Sweden’s number one model
2 . first on a list of several things to be considered, done etc:
item number one on the agenda
This has got to be our number one task.