Source : Webster's Revised Unabridged Dictionary (1913)

Why \Why\, n.
   A young heifer. [Prov. Eng.] --Grose.

Why \Why\, adv. [OE. whi, why, AS. hw[=i], hw?, instrumental
   case of hw[=a], hw[ae]t; akin to Icel. hv[=i] why, Dan. & Sw.
   hvi; cf. Goth. hw?. ?. See {Who}.]
   1. For what cause, reason, or purpose; on what account;
      wherefore; -- used interrogatively. See the Note under
      {What}, pron., 1.

            Turn ye, turn ye from your evil ways; for why will
            ye die, O house of Israel?            --Ezek.
                                                  xxxiii. 11.

   2. For which; on account of which; -- used relatively.

            No ground of enmity between us known Why he should
            mean me ill or seek to harm.          --Milton.

            Turn the discourse; I have a reason why I would not
            have you speak so tenderly.           --Dryden.

   3. The reason or cause for which; that on account of which;
      on what account; as, I know not why he left town so
      suddenly; -- used as a compound relative.

   Note: Why is sometimes used as an interjection or an
         expletive in expression of surprise or content at a
         turn of affairs; used also in calling. ``Why,
         Jessica!'' --Shak.

               If her chill heart I can not move, Why, I'll
               enjoy the very love.               --Cowley.
         Sometimes, also, it is used as a noun.

               The how and the why and the where. --Goldsmith.

   {For why}, because; why. See {Forwhy}. [Obs. or Colloq.]

Source : WordNet®

why
     n : the cause or intention underlying an action or situation,
         especially in the phrase `the whys and wherefores' [syn:
         {wherefore}]
     adv : question word; what is the reason (`how come' is a
           nonstandard variant); "why are you here?"; "how come he
           got an ice cream cone but I didn't?" [syn: {how come}]