I. noun
Date: circa 1705
1. : one's own interests or welfare : oneself
looking out for number one
— often written No. 1
2. : one that is first in rank, importance, or influence — often written No. 1
II. adjective
Date: 1839
1. : first in rank, importance, or influence : foremost
cancer is the country's number one killer
— often written No. 1
2. : of highest or of high quality