Source : Webster's Revised Unabridged Dictionary (1913)

Thinker \Think"er\, n.
   One who thinks; especially and chiefly, one who thinks in a
   particular manner; as, a close thinker; a deep thinker; a
   coherent thinker.

Source : WordNet®

thinker
     n 1: an important intellectual; "the great minds of the 17th
          century" [syn: {creative thinker}, {mind}]
     2: someone who exercises the mind (usually in an effort to
        reach a decision)