An undocumented feature is an unintended or undocumented hardware operation, for example an undocumented instruction, or software feature found in computer hardware and software that is considered beneficial or useful. Sometimes the documentation is omitted through oversight, but undocumented features are sometimes not intended for use by end users, but left available for use by the vendor for software support and development. Also, some unintended operation of hardware or software that ends up being of utility to users is simply a bug, flaw or quirk.

Since the suppliers of the software usually consider the software documentation to constitute a contract for the behavior of the software, undocumented features are generally left unsupported and may be removed or changed at will and without notice to the users.

Some user-reported defects are viewed by software developers as working as expected, leading to the catchphrase "it's not a bug, it's a feature" (INABIAF) and its variations.