2 Answers

If you can understand this, it may give you your answer. I could not find an actual definition, this article seems to at least describe what a breakpoint is: 

"We report here the breakpoint structure and sequences of the Drosophila melanogaster cosmopolitan chromosomal inversion In(3R)P. Combining in situ hybridization to polytene chromosomes and long-range PCR, we have identified and sequenced the distal and proximal breakpoints. The breakpoints are not simple cut-and-paste structures; gene fragments and small duplications of DNA are associated with both breaks. The distal breakpoint breaks the tolkin (tok) gene and the proximal breakpoint breaks CG31279 and the tolloid (tld) gene. Functional copies of all three genes are found at the opposite breakpoints." 

Source: http://www.questia.com/library/1P3-1035665781/the-structure-and-population-genetics-of-the-breakpoints 

[CG31279]