From The Collaborative International Dictionary of English v.0.48:

1 \1\ adj.
   1. used of a single unit or thing; not two or more; --
      representing the number one as an Arabic numeral.

   Syn: one, i, ane
        [WordNet 1.5 +PJC]

From WordNet (r) 2.0:

1
     adj : used of a single unit or thing; not two or more; "`ane' is
           Scottish" [syn: one, i, ane]
     n : the smallest whole number or a numeral representing this
         number; "he has the one but will need a two and three to
         go with it"; "they had lunch at one" [syn: one, I, ace,
          single, unity]