The Collaborative International Dictionary of English v.0.48:

Stet \Stet\ (st[e^]t), L., subj. 3d pers. sing. of stare to
   stand, remain. [See Stand.] (Print.)
   Let it stand; -- a word used by proof readers to signify that
   something once erased, or marked for omission, is to remain.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Stet \Stet\, v. t. [imp. & p. p. Stetted; p. pr. & vb. n.
   Stetting.] (Print.)
   To cause or direct to remain after having been marked for
   omission; to mark with the word stet, or with a series of
   dots below or beside the matter; as, the proof reader stetted
   a deled footnote.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stet
    v 1: printing: cancel, as of a correction or deletion
    2: printing: direct that a matter marked for omission or
       correction is to be retained (used in the imperative)