The Collaborative International Dictionary of English v.0.48:

Undulate \Un"du*late\, a. [L. undulatus undulated, wavy, a dim.
   from unda a wave; cf. AS. ??, Icel. unnr; perhaps akin to E.
   water. Cf. Abound, Inundate, Redound, Surround.]
   Same as Undulated.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Undulate \Un"du*late\, v. t. [imp. & p. p. Undulated; p. pr. &
   vb. n. Undulating.]
   To cause to move backward and forward, or up and down, in
   undulations or waves; to cause to vibrate.
   [1913 Webster]

         Breath vocalized, that is, vibrated and undulated.
                                                  --Holder.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Undulate \Un"du*late\, v. i.
   To move in, or have, undulations or waves; to vibrate; to
   wave; as, undulating air.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undulate
    adj 1: having a wavy margin and rippled surface
    v 1: stir up (water) so as to form ripples [syn: ripple,
         ruffle, riffle, cockle, undulate]
    2: occur in soft rounded shapes; "The hills rolled past" [syn:
       roll, undulate]
    3: move in a wavy pattern or with a rising and falling motion;
       "The curtains undulated"; "the waves rolled towards the
       beach" [syn: roll, undulate, flap, wave]
    4: increase and decrease in volume or pitch, as if in waves;
       "The singer's voice undulated"

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "undulate":
   alternate, bank, be here again, be poised, billow, brandish, break,
   circle, comb, come again, come and go, come around, come round,
   come round again, come up again, crab, crash, cycle, dash, dip,
   ebb and flow, feather, fishtail, flap, flaunt, float, flourish,
   flutter, fly, heave, intermit, lift, loop, oscillate, peak, plow,
   popple, porpoise, pull out, pull up, pulsate, pulse, push down,
   reappear, recur, reoccur, repeat, return, revolve, rise,
   rise and fall, roll, roll around, rotate, scend, send, shake,
   sideslip, skid, slidder, smash, snake, spin, spiral, stunt, surge,
   swell, swing, toss, turn, wag, wave, wheel, wheel around, wield,
   wigwag, yaw