The Collaborative International Dictionary of English v.0.48:

Admonition \Ad`mo*ni"tion\, n. [OE. amonicioun, OF. amonition,
   F. admonition, fr. L. admonitio, fr. admonere. See
   Admonish.]
   Gentle or friendly reproof; counseling against a fault or
   error; expression of authoritative advice; friendly caution
   or warning.
   [1913 Webster]

   Syn: Admonition, Reprehension, Reproof.

   Usage: Admonition is prospective, and relates to moral
          delinquencies; its object is to prevent further
          transgression. Reprehension and reproof are
          retrospective, the former being milder than the
          latter. A person of any age or station may be liable
          to reprehension in case of wrong conduct; but reproof
          is the act of a superior. It is authoritative
          fault-finding or censure addressed to children or to
          inferiors.
          [1913 Webster]

WordNet (r) 3.0 (2006):

admonition
    n 1: cautionary advice about something imminent (especially
         imminent danger or other unpleasantness); "a letter of
         admonition about the dangers of immorality"; "the warning
         was to beware of surprises"; "his final word of advice was
         not to play with matches" [syn: admonition, monition,
         warning, word of advice]
    2: a firm rebuke [syn: admonition, admonishment, monition]

The Devil's Dictionary (1881-1906):

ADMONITION, n.  Gentle reproof, as with a meat-axe.  Friendly warning.

    Consigned by way of admonition,
    His soul forever to perdition.
                                                              Judibras