Longman Dictionary of Contemporary English homepage

number one

1 noun
     
Related topics: Music
number one1
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.
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

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

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