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. make, do, or go number one, Informal.to urinate: used esp. by or with reference to children.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.