The Collaborative International Dictionary of English v.0.48:

Billow \Bil"low\, n. [Cf. Icel. bylgja billow, Dan. b["o]lge,
   Sw. b["o]lja; akin to MHG. bulge billow, bag, and to E.
   bulge. See Bulge.]
   1. A great wave or surge of the sea or other water, caused
      usually by violent wind.
      [1913 Webster]

            Whom the winds waft where'er the billows roll.
                                                  --Cowper.
      [1913 Webster]

   2. A great wave or flood of anything. --Milton.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Billow \Bil"low\, v. i. [imp. & p. p. Billowed; p. pr. & vb.
   n. Billowing.]
   To surge; to rise and roll in waves or surges; to undulate.
   "The billowing snow." --Prior.
   [1913 Webster]

WordNet (r) 3.0 (2006):

billow
    n 1: a large sea wave [syn: billow, surge]
    v 1: rise up as if in waves; "smoke billowed up into the sky"
         [syn: billow, wallow]
    2: move with great difficulty; "The soldiers billowed across the
       muddy riverbed"
    3: rise and move, as in waves or billows; "The army surged
       forward" [syn: billow, surge, heave]
    4: become inflated; "The sails ballooned" [syn: balloon,
       inflate, billow]