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"