Source : Webster's Revised Unabridged Dictionary (1913)

Formication \For`mi*ca"tion\, n. [L. formicatio, fr. formicare
   to creep like an ant, to feel as if ants were crawling on
   one's self, fr. formica ant: cf. F. formication.] (Med.)
   A sensation resembling that made by the creeping of ants on
   the skin. --Dunglison.

Source : WordNet®

formication
     n : hallucinated sensation that insects or snakes are crawling
         over the skin; a common side-effect of extensive use of
         cocaine or amphetamines