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]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "billow":
   bag, balloon, be poised, belly, belly out, bilge, bore, bouge,
   break, breakers, bug, bulge, chop, choppiness, chopping sea, comb,
   comber, crash, dash, dilate, dirty water, distend, eagre,
   ebb and flow, goggle, gravity wave, ground swell, heave, heavy sea,
   heavy swell, lift, lop, peak, pooch, pop, popple, pouch, pout,
   riffle, ripple, rise, rise and fall, roll, roller, rough water,
   round out, scend, sea, send, smash, surf, surge, swell, swell out,
   tidal bore, tidal wave, tide wave, toss, trough, tsunami, undulate,
   undulation, water wave, wave, wavelet, white horses, whitecaps