noun , adjective
( informal )
■ noun
1.
[ U ] the most important or best person or thing :
We're number one in the used car business.
2.
[ U , C ] the pop song or record 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.
[ U ] yourself :
Looking after number one is all she thinks about.
4.
[ sing. ] 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
■ adjective
most important or best :
the world's number one athlete
the number one priority