The Collaborative International Dictionary of English v.0.48:

Alternate \Al*ter"nate\ (?; 277), a. [L. alternatus, p. p. of
   alternate, fr. alternus. See Altern, Alter.]
   1. Being or succeeding by turns; one following the other in
      succession of time or place; by turns first one and then
      the other; hence, reciprocal.
      [1913 Webster]

            And bid alternate passions fall and rise. --Pope.
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   2. Designating the members in a series, which regularly
      intervene between the members of another series, as the
      odd or even numbers of the numerals; every other; every
      second; as, the alternate members 1, 3, 5, 7, etc.; read
      every alternate line.
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   3. (Bot.) Distributed, as leaves, singly at different heights
      of the stem, and at equal intervals as respects angular
      divergence. --Gray.
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   Alternate alligation. See Alligation.

   Alternate angles (Geom.), the internal and angles made by
      two lines with a third, on opposite sides of it. It the
      parallels AB, CD, are cut by the line EF, the angles AGH,
      GHD, as also the angles BGH and GHC, are called alternate
      angles.

   Alternate generation. (Biol.) See under Generation.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Alternate \Al"ter*nate\ (?; 277), v. t. [imp. & p. p.
   Alternated; p. pr. & vb. n. Alternating.] [L. alternatus,
   p. p. of alternare. See Altern.]
   To perform by turns, or in succession; to cause to succeed by
   turns; to interchange regularly.
   [1913 Webster]

         The most high God, in all things appertaining unto this
         life, for sundry wise ends alternates the disposition
         of good and evil.                        --Grew.
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The Collaborative International Dictionary of English v.0.48:

Alternate \Al"ter*nate\, v. i.
   1. To happen, succeed, or act by turns; to follow
      reciprocally in place or time; -- followed by with; as,
      the flood and ebb tides alternate with each other.
      [1913 Webster]

            Rage, shame, and grief alternate in his breast. --J.
                                                  Philips.
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            Different species alternating with each other.
                                                  --Kirwan.
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   2. To vary by turns; as, the land alternates between rocky
      hills and sandy plains.
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The Collaborative International Dictionary of English v.0.48:

Alternate \Al*ter"nate\ (?; 277), n.
   1. That which alternates with something else; vicissitude.
      [R.]
      [1913 Webster]

            Grateful alternates of substantial.   --Prior.
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   2. A substitute; one designated to take the place of another,
      if necessary, in performing some duty.
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   3. (Math.) A proportion derived from another proportion by
      interchanging the means.
      [1913 Webster]

WordNet (r) 3.0 (2006):

alternate
    adj 1: every second one of a series; "the cleaning lady comes on
           alternate Wednesdays"; "jam every other day"- the White
           Queen
    2: serving or used in place of another; "an alternative plan"
       [syn: alternate, alternative, substitute]
    3: occurring by turns; first one and then the other;
       "alternating feelings of love and hate" [syn: alternate(a),
       alternating(a)]
    4: of leaves and branches etc; first on one side and then on the
       other in two ranks along an axis; not paired; "stems with
       alternate leaves" [ant: opposite, paired]
    n 1: someone who takes the place of another person [syn:
         surrogate, alternate, replacement]
    v 1: go back and forth; swing back and forth between two states
         or conditions [syn: alternate, jump]
    2: exchange people temporarily to fulfill certain jobs and
       functions
    3: be an understudy or alternate for a role [syn: understudy,
       alternate]
    4: reverse (a direction, attitude, or course of action) [syn:
       interchange, tack, switch, alternate, flip, flip-
       flop]
    5: do something in turns; "We take turns on the night shift"
       [syn: alternate, take turns]

Moby Thesaurus II by Grady Ward, 1.0:

232 Moby Thesaurus words for "alternate":
   advocate, agent, alter ego, alternating, alternative,
   amicus curiae, analogy, attorney, back and fill, back-and-forth,
   backup, backup man, bandy, battledore and shuttlecock,
   be here again, be quits with, beating, champion, change,
   changeling, circle, circling, come again, come and go, come around,
   come round, come round again, come up again, commute, comparison,
   compensate, complementary, cooperate, copy, corresponding,
   counterchange, counterfeit, cover, cycle, cyclic, deputy, dither,
   do a hitch, do a stint, do a tour, do time, double, dummy,
   ebb and flow, enlist, epochal, equal, equivalent, equivocate,
   ersatz, even, every other, exchange, exchangeable,
   executive officer, exponent, fake, figurehead, fill in for,
   fill-in, flounder, fluctuate, get back at, get even with, ghost,
   ghostwriter, give and take, go through phases, have a go,
   have tenure, hitch and hike, hold office, imitation, in rotation,
   interchange, interchangeable, intermit, intermittent, isochronal,
   isochronous, keep a watch, lieutenant, locum, locum tenens,
   logroll, makeshift, measured, metaphor, metonymy, metronomic, mock,
   next best thing, oscillate, oscillatory, other, paranymph,
   pass and repass, pay back, pendulate, periodic, periodical,
   permute, personnel, phony, pinch, pinch hitter, pleader,
   procurator, provisional, proxy, pulsate, pulse, pulsing, re-up,
   reappear, reciprocal, reciprocate, reciprocative, recur, recurrent,
   recurring, reenlist, relief, relieve, reoccur, repeat, replacement,
   replacing, representative, requite, reserve, reserves, respond,
   retaliate, return, return the compliment, revolve, rhythmic,
   ride and tie, ring the changes, ringer, roll around, rotary,
   rotate, rotating, seasonal, second, second in command,
   second string, secondary, seesaw, serial, serve time, shift,
   shilly-shally, shuffle, shuttle, shuttlecock, sign, sign up,
   sine wave, spare, spares, spell, spell off, stagger, stand-in,
   steady, stopgap, sub, substituent, substitute, substitution,
   succedaneum, succeed, successive, superseder, supplanter,
   surrogate, swap, sway, swing, switch, symbol, synecdoche,
   take turns, teeter, teeter-totter, temporary, tentative,
   tergiversate, third string, time off, to-and-fro, token, totter,
   trade, transpose, turn, understudy, undulant, undulate, undulatory,
   up-and-down, utility, utility man, utility player, vacillate,
   variant, vary, vicar, vicar general, vicarious, vice,
   vice-president, vice-regent, vicegerent, wavelike, waver,
   wax and wane, wheel, wheel around, wheeling, wibble-wabble, wigwag,
   wobble, zigzag