A type of finger ring of no generally standardized form that is given by a man to his fiancée as a token of betrothal, usually called today an engagement ring.
Such rings have been used since Roman times (called anulus pronubus), when they were originally made of iron (a gold ring was generally forbidden) and without any gemstone; but later gold rings were so used, bearing appropriate amatory inscriptions, as well as motifs such as a lover's knot or clasped hands and set with gemstones.
When more significance became attached to betrothal, the ring became known as an engagement ring, and it was also used as a wedding ring, without any special change in form or style.
Other forms of finger rings may have been used as a betrothal ring, e.g. gimmel (gemel) ring and puzzle ring.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson