The Collaborative International Dictionary of English v.0.48:

Ditto \Dit"to\, adv.
   As before, or aforesaid; in the same manner; also.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ditto \Dit"to\, n.; pl. Dittos. [It., detto, ditto, fr. L.
   dictum. See Dictum.]
   The aforesaid thing; the same (as before). Often contracted
   to do., or to two "turned commas" ("), or small marks. Used
   in bills, books of account, tables of names, etc., to save
   repetition.
   [1913 Webster]

         A spacious table in the center, and a variety of
         smaller dittos in the corners.           --Dickens.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ditto
    n 1: a mark used to indicate the word above it should be
         repeated [syn: ditto mark, ditto]
    v 1: repeat an action or statement; "The next speaker dittoed
         her argument"

Moby Thesaurus II by Grady Ward, 1.0:

178 Moby Thesaurus words for "ditto":
   Doppelganger, Photostat, Xerox, accord, act like, actual thing,
   affect, afresh, again, agree, agree in opinion, agree with, alike,
   amount to, anew, as is, assume, balance, bis, bob, borrow,
   break even, burden, carbon, carbon copy, chant, chime in with,
   chorus, clone, close with, coequal, coequally, coextensively,
   coincide, coincidentally, come again, come to, come up to, compeer,
   concur, conform to, congruently, copy, correspond, correspondently,
   correspondingly, coterminously, counterfeit, counterpart, crib,
   da capo, de novo, dead ringer, do, do a repeat, do again, do like,
   do over, double, draw, dupe, duplicate, duplication, echo, encore,
   equal, equally, equipollent, equivalent, even, even off,
   exact counterpart, facsimile, fake, fall in with, fellow, forge,
   from the beginning, geminate, go along with, go like, go with,
   hectograph, hoke, hoke up, homograph, homonym, homophone, ibid,
   ibidem, idem, identical same, identically, imitate, ingeminate,
   just the same, keep pace with, knot, like, likewise, make like,
   manifold, match, match up with, mate, measure up to, meet,
   microcopy, microfilm, mimeo, mimeograph, mirror, model, multigraph,
   multiply by two, no other, none other, once more, opposite number,
   over, over again, parallel, parrot, peer, plagiarize,
   quadruplicate, quote, reach, redo, redouble, reduplicate,
   reduplication, reecho, reflect, refrain, regurgitate, reincarnate,
   renew, repeat, repetend, repetition, replica, replicate,
   replication, representation, reproduce, reproduction, revive,
   ritornello, rival, run abreast, run to, same here, say again,
   selfsame, side with, simulate, spit and image, spitting image,
   stack up with, stat, strike in with, synonym, synonymously,
   the same, the same difference, the same way, tie, touch, trace,
   transcribe, triplicate, twice over, twin, two times, undersong,
   very image, very same