The Collaborative International Dictionary of English v.0.48:

Partisan \Par"ti*san\ (p[aum]r"t[i^]*zan), n. [F., fr. It.
   partigiano. See Party, and cf. Partisan a truncheon.]
   [Written also partizan.]
   1. An adherent to a party or faction; esp., one who is
      strongly and passionately devoted to a party or an
      interest. "The violence of a partisan." --Macaulay.
      [1913 Webster]

            Both sides had their partisans in the colony.
                                                  --Jefferson.
      [1913 Webster]

   2. (Mil.)
      (a) The commander of a body of detached light troops
          engaged in making forays and harassing an enemy.
      (b) Any member of such a corps.
          [1913 Webster]

WordNet (r) 3.0 (2006):

partizan
    adj 1: devoted to a cause or party [syn: partisan, partizan]
           [ant: nonpartisan, nonpartizan]
    n 1: an ardent and enthusiastic supporter of some person or
         activity [syn: enthusiast, partisan, partizan]
    2: a pike with a long tapering double-edged blade with lateral
       projections; 16th and 17th centuries [syn: partisan,
       partizan]