oral, verbal, aural
People talk of verbal contracts when they mean things agreed by word of mouth, not written down.
Strictly speaking that’s not the right word. Verbal means “of or relating to words” (whether spoken or written). Technically, a verbal contract might be an agreement entered into by word of mouth or in writing or both.
A more correct word to describe an unwritten agreement is oral. Oral means “of or relating to the mouth” as in oral hygiene, or “spoken rather than written” as in an oral contract.
In education you get oral exams in various subjects – especially in languages, for which it’s important to test a candidate’s speaking skills. Not to be confused with the similar-sounding aural tests. Aural means “relating to the ear or hearing”. An aural test assesses a musician’s ability to listen to a piece of music by playing or singing it back or by describing it to the examiner.
You can read about the difference between oral and verbal and a whole lot more, in the Clifford & Co editorial style guide.