The Collaborative International Dictionary of English v.0.48:

Collectivist \Col*lect"iv*ist\, n. [Cf. F. collectiviste.]
   An advocate of collectivism.
   [WordNet 1.5] Collectivist

The Collaborative International Dictionary of English v.0.48:

Collectivist \Col*lect"iv*ist\, collectivistic
\col*lec`tiv*is"tic\a.
   1. Relating to, characteristic of, or advocating,
      collectivism.
      [1913 Webster]

   2. operating under collectivist principles.

   Note: [Narrower terms: socialistic (vs. capitalistic),
         socialist; communistic]

   Syn: collectivized, state-controlled.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

collectivist
    adj 1: subscribing to the socialistic doctrine of ownership by
           the people collectively [syn: collectivist,
           collectivistic, collectivized, collectivised,
           state-controlled]
    n 1: a person who belongs to the political left [syn:
         collectivist, leftist, left-winger]