Source : WordNet®

number one
     n 1: a reference to yourself or myself etc.; `take care of number
          one' means to put your own interests first
     2: the first or highest in an ordering or series; "He wanted to
        be the first" [syn: {first}]
     3: the first element in a countable series; "the first of the
        month" [syn: {first}, {number 1}]