The Collaborative International Dictionary of English v.0.48:

Regimen \Reg"i*men\ (r?j"?*m?n), n. [L. regimen, -inis, fr.
   regere to guide, to rule. See Right, and cf. Regal,
   R['e]gime, Regiment.]
   1. Orderly government; system of order; adminisration.
      --Hallam.
      [1913 Webster]

   2. Any regulation or remedy which is intended to produce
      beneficial effects by gradual operation; esp. (Med.), a
      systematic course of diet, etc., pursed with a view to
      improving or preserving the health, or for the purpose of
      attaining some particular effect, as a reduction of flesh;
      -- sometimes used synonymously with hygiene.
      [1913 Webster]

   3. (Gram.)
      (a) A syntactical relation between words, as when one
          depends on another and is regulated by it in respect
          to case or mood; government.
      (b) The word or words governed.
          [1913 Webster]

WordNet (r) 3.0 (2006):

regimen
    n 1: (medicine) a systematic plan for therapy (often including
         diet) [syn: regimen, regime]