The Collaborative International Dictionary of English v.0.48:

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.
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            'T is dangerous tampering with a muse. --Roscommon.
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   2. To meddle so as to alter, injure, or vitiate a thing.
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   3. To deal unfairly; to practice secretly; to use bribery.
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            Others tampered
            For Fleetwood, Desborough, and Lambert. --Hudibras.
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WordNet (r) 3.0 (2006):

tampering
    n 1: the act of altering something secretly or improperly [syn:
         meddling, tampering]