On March 7, 2008, Microsoft formally defined what an exploit is:

"An exploit is a form of software that replaces or modifies authentic 
Windows components."

I made a copy of the KB Article, 940510.  It's at:

http://www.stat.colostate.edu/~zube/docs/940510.html

and the definition is the second sentence.

zube@stat.colostate.edu