The Collaborative International Dictionary of English v.0.48:

Twice \Twice\ (tw[imac]s), adv. [OE. twies (where the s is the
   adverbial ending; see -wards), twie, AS. twiges, twiwa;
   akin to twi- (in comp.) two, G. zwie-, OHG. zwi-, Icel.
   tv[imac]-, L. bi-, Gr. di-, Skr. dvi-, and E. two. See
   Two.]
   [1913 Webster]
   1. Two times; once and again.
      [1913 Webster]

            He twice essayed to cast his son in gold. --Dryden.
      [1913 Webster]

   2. Doubly; in twofold quantity or degree; as, twice the sum;
      he is twice as fortunate as his neighbor.
      [1913 Webster]

   Note: Twice is used in the formation of compounds, mostly
         self-explaining; as, twice-born, twice-conquered,
         twice-planted, twice-told, and the like.
         [1913 Webster]

WordNet (r) 3.0 (2006):

twice
    adv 1: two times; "I called her twice"
    2: to double the degree; "she was doubly rewarded"; "his eyes
       were double bright" [syn: doubly, double, twice]

The Devil's Dictionary (1881-1906):

TWICE, adv.  Once too often.