Source : Webster's Revised Unabridged Dictionary (1913)

Tamper \Tam"per\, v. i. [imp. & p. p. {Tampered}; p. pr. & vb.
   n. {Tampering}.] [A corruption of temper.]
   1. To meddle; to be busy; to try little experiments; as, to
      tamper with a disease.

            'T is dangerous tampering with a muse. --Roscommon.

   2. To meddle so as to alter, injure, or vitiate a thing.

   3. To deal unfairly; to practice secretly; to use bribery.

            Others tampered For Fleetwood, Desborough, and
            Lambert.                              --Hudibras.

Source : WordNet®

tampering
     n : the act of altering something secretly or improperly [syn: {meddling}]