The Collaborative International Dictionary of English v.0.48:

break dancing \break" danc`ing\ n.
   a type of solo dancing in which the dancer performs elaborate
   footwork, pantomime, and various acrobatic movements, such as
   spinning head stands or spinning on the back, and usually
   containing much improvization. It was first performed by male
   teenagers in the 1970's, in small informal groups on the
   streets, and is often performed to the accompaniment of rap
   music or other rock music with a rapid, strong beat. --
   break" dance`, n. -- break" dan`cer, n.
   [PJC]

WordNet (r) 3.0 (2006):

break dance
    n 1: a form of solo dancing that involves rapid acrobatic moves
         in which different parts of the body touch the ground;
         normally performed to the rhythm of rap music [syn: break
         dancing, break dance]
    v 1: do a break dance; "Kids were break-dancing at the street
         corner" [syn: break dance, break-dance, break]