The Collaborative International Dictionary of English v.0.48:

Luck \Luck\, n. [Akin to D. luk, geluk, G. gl["u]ck, Icel.
   lukka, Sw. lycka, Dan. lykke, and perh. to G. locken to
   entice. Cf. 3d Gleck.]
   That which happens to a person; an event, good or ill,
   affecting one's interests or happiness, and which is deemed
   casual; a course or series of such events regarded as
   occurring by chance; chance; hap; fate; fortune; often, one's
   habitual or characteristic fortune; as, good, bad, ill, or
   hard luck. Luck is often used by itself to mean good luck;
   as, luck is better than skill; a stroke of luck.
   [1913 Webster]

         If thou dost play with him at any game,
         Thou art sure to lose; and of that natural luck,
         He beats thee 'gainst the odds.          --Shak.
   [1913 Webster]

   Luck penny, a small sum given back for luck to one who pays
      money. [Prov. Eng.]

   To be in luck, to receive some good, or to meet with some
      success, in an unexpected manner, or as the result of
      circumstances beyond one's control; to be fortunate.
      [1913 Webster]

WordNet (r) 3.0 (2006):

luck
    n 1: your overall circumstances or condition in life (including
         everything that happens to you); "whatever my fortune may
         be"; "deserved a better fate"; "has a happy lot"; "the luck
         of the Irish"; "a victim of circumstances"; "success that
         was her portion" [syn: fortune, destiny, fate,
         luck, lot, circumstances, portion]
    2: an unknown and unpredictable phenomenon that causes an event
       to result one way rather than another; "bad luck caused his
       downfall"; "we ran into each other by pure chance" [syn:
       luck, fortune, chance, hazard]
    3: an unknown and unpredictable phenomenon that leads to a
       favorable outcome; "it was my good luck to be there"; "they
       say luck is a lady"; "it was as if fortune guided his hand"
       [syn: luck, fortune]