number one

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that is first in rank, order, or importance.
• noun Slang One's own interests; oneself.
• noun The act of urinating.
• noun Urine.
• adjective First in rank, order, or importance.
• adjective Foremost in quality; first-rate.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective idiomatic First; foremost; best.
• noun the most important person, the one who is in charge
• noun someone who is top of a ranking, who is ranked first.
• noun oneself, being considered foremost, as by an egoist
• noun Urine; urination.
• noun music the single that has sold the most in a given period.
• noun soccer The main goalkeeper of a team, so-called because they wear the number 1 on the back of their kit.
• noun cricket The batsman who opens the batting.
• noun Used other than as an idiom: see number,‎ one.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a reference to yourself or myself etc.; `take care of number one' means to put your own interests first
• noun the first element in a countable series
• noun the first or highest in an ordering or series