attested

Proven; shown to be true with evidence.

Supported with testimony.

Certified as good, correct, or pure.

A Contract of eternall bond of loue, Confirm'd by mutuall ioynder of your hands, Atteſted by the holy cloſe of lippes,

Of words or languages, proven to have existed by records.

The word "slæpwerig" (sleep-weary) is attested in the Exeter Book in the form "slæpwerigne".

A term should be included if it's likely that someone would run across it and want to know what it means. This in turn leads to the somewhat more formal guideline of including a term if it is attested and idiomatic.