The Collaborative International Dictionary of English v.0.48:

fine-tune \fine-tune\ v. t.
   to adjust carefully and precisely so as to achieve optimum
   performance or efficiency; as, The mechanic fine-tuned the
   engine.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

fine-tune
    v 1: improve or perfect by pruning or polishing; "refine one's
         style of writing" [syn: polish, refine, fine-tune,
         down]
    2: adjust finely; "fine-tune the engine" [syn: fine-tune,
       tweak]
    3: make fine adjustments or divide into marked intervals for
       optimal measuring; "calibrate an instrument"; "graduate a
       cylinder" [syn: calibrate, graduate, fine-tune]