attested

oikeaksi todistettu, vahvistettu (en-past of)
proven Proven; shown to be true with evidence
supported Supported with testimony
certified Certified as good, correct, or pure
1599, w:William Shakespeare|William Shakespeare, s:Twelfth Night|Twelfth Night, First Folio edition, Act V, Scene 1:

A Contract of eternall bond of loue,

Confirm'd by mutuall ioynder of your hands,

Atteſted by the holy cloſe of lippes,

A term should be included if it's likely that someone would run accross 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.