Thirteen years? Really?
People insist that "X, by definition, is a Y!" on those occasions when they're trying to sneak in a connotation of Y that isn't directly in the definition, and X doesn't look all that much like other members of the Y cluster.
Over the last thirteen years I've been keeping track of how often this phrase is used correctly versus incorrectly - though not with literal statistics, I fear. But eyeballing suggests that using the phrase by definition, anywhere outside of math, is among the most alarming signals of flawed rgument I've ever found. It's right up there with "Hitler", "God", "absolutely certain" and "can't prove that".
I think I found the Feynman usage. Tried to leave that in a comment, but typepad crapped out on me. Also, the author is guilty of using two spaces after a period so any conclusions must, by definition, be ignored.