can anyone please enlighten me. how does the Catholic Church define adultery…
Catholic Encyclopedia: Adultery
Adultery is defined as carnal connection between a married person and one unmarried, or between a married person and the spouse of another. It is seen to differ from fornication in that it supposes the marriage of one or both of the agents. Nor is it necessary that this marriage be already consummated; it need only be what theologians call matrimonium ratum. Sexual commerce with one engaged to another does not, it is most generally held, constitute adultery. Again, adultery, as the definition declares, is committed in carnal intercourse. Nevertheless immodest actions indulged in between a married person and another not the lawful spouse, while not of the same degree of guilt, are of the same character of malice as adultery
Catechism of the Catholic Church: The Sixth Commandment
2380 Adultery refers to marital infidelity. When two partners, of whom at least one is married to another party, have sexual relations - even transient ones - they commit adultery. Christ condemns even adultery of mere desire.171 The sixth commandment and the New Testament forbid adultery absolutely.172 The prophets denounce the gravity of adultery; they see it as an image of the sin of idolatry.173
I think there are really two aspects we need to cover.
As already mentioned with the definitions and references above adultery, strictly speaking, is sexual activity by or with a married person and someone who isn’t their spouse.
However, in the broader context, the Church understands the sixth commandment to be all encompassing of sins against chastity and marriage, not just strictly the one sin of “adultery”. Just as the fifth commandment encompasses all sins against life, not just strictly one sin of “murder”.
Thank you very much for clearing that up for me.