Of course this all depends on jurisdiction, but e.g. in the US for example fair use does not show up [oops, see note] in the limitations and exceptions in the copyright definition (like e.g. government use does), but relies on case law, the "fair use doctrine". This means quite literally it is a framework for understanding the defence of a particular use based on precedent: the defendant(s) argued it.

It's a bit of a quibble to differentiate between allowable infringement or exception, here, so i've edited original.

[note: 0xcde4c3db points out correctly that it does actually show up since 1976, but in a way the defers much of the definition back to case law, so same effect]