number one

WordReference Random House Unabridged Dictionary of American English © 2025

num′ber one′, 

1. oneself, esp. one's own well-being or interests:
   to look out for number one.
2. a person, company, etc., that is first in rank, order, or prestige:
   Our team is number one.
3. of the highest in quality, status, importance, etc.;
   first-rate:
   a number one performance.
4. first in rank, order, or prestige:
   the number one book on the bestseller list.
5. urination: used esp. by or with reference to children.
6. Idioms make, do, or go number one, [Informal.] to urinate: used esp. by or with reference to children.
• 1830–40

Collins Concise English Dictionary © HarperCollins Publishers::

number one noun

1. the first in a series or sequence
2. an informal phrase for oneself, myself: to look after number one
3. the most important person; leader, chief: a new number one in the organization
4. the bestselling pop record in any one week

adjective

1. first in importance, urgency, quality, etc: number one priority