Robert Ludlum wrote 27 books, and almost all of them are titled The Something Something. For example: The Scarlatti Inheritance, The Parsifal Mosaic, The Holcroft Covenant, The Woodchuck Infestation, The Camembert Flatulence. Okay, not those last two—but you get the idea.
The Bourne Identity uses that old The Something Something formula, but it's a little trickier than some of the other Something Somethings. The Bourne Identity means the identity Bourne has lost, the self he's trying to find again. The Bourne identity here is David Webb. But, as we read, we also discover that the Bourne identity isn't just who Bourne is, it's the disguise he's taken to fool Carlos. In addition the Bourne identity (David Webb) is the life and the name that were taken: it's the traitor named Bourne whom "Bourne" killed in Vietnam. So the Bourne identity is who Bourne is, who he is not, and who his enemy and victim were as well.
Finally, an identity can refer not only to a self or an assumed persona, but also to an equation. The mathematical phrase x=y, for example, is an identity. Bourne often repeats his various names or identities as if they're a mathematical equations: "Cain is for Charlie and Delta if for Cain" (29.61). The title The Bourne Identity could, then, refer not to one self, either real or fake, but to the way Bourne slides from one to the other, so that there is not one name, but many names, each identical to, or identifying, the next. The truest Bourne identity, from this perspective, is a series of masks.