Source : Webster's Revised Unabridged Dictionary (1913)

Overestimate \O`ver*es"ti*mate\, v. t.
   To estimate too highly; to overvalue.

Overestimate \O`ver*es"ti*mate\, n.
   An estimate that is too high; as, an overestimate of the
   vote.

Source : WordNet®

overestimate
     n 1: an appraisal that is too high [syn: {overestimation}, {overvaluation},
           {overappraisal}]
     2: a calculation that results in an estimate that is too high
        [syn: {overestimation}, {overrating}, {overreckoning}]
     v 1: make too high an estimate of; "He overestimated his own
          powers" [syn: {overrate}] [ant: {underestimate}]
     2: assign too high a value to; "You are overestimating the
        value of your old car" [syn: {overvalue}] [ant: {undervalue},
         {undervalue}]