What is an "enforceability defense"?

An "enforceability defense" is a defense to patent infringement which, if successfully maintained, will result in a holding that the patent in question is unenforceable (although this is not technically the same thing as the patent being held "invalid." Invalidity concerns one or more claims of the patent failing the substantive legal requirements of patent law; Unenforceability relates to the patentee's own conduct).   If a patent is unenforceable, it cannot thereafter be used as the basis of an infringement claim. I will cover specific enforceability defenses (e.g., inequitable conduct, patent misuse, etc.) in subsequent posts.