Parashas Vayeira · Passuk 20:3
And God came to Avimelech in a dream of the night and said to him, “Behold, you are a dead man because of the woman whom you have taken, for she is a man’s wife.”
Torah Temimah points to Yerushalmi Kiddushin 1:1.
TT lemma: בעולת בעל.
Sefaria — Yerushalmi Kiddushin
English: “A man’s wife.” Rabbi Abbahu taught in the name of Rabbi Elazar: “And she is a man’s wife”—he is liable for married women, but he is not liable for betrothed women.
Derash on the verse (Yerushalmi) — not a formal named middah of the 13. The phrase distinguishes the stages of marriage for Noachide law.
Under the laws of the children of Noach, liability for adultery attaches only to fully married women, not to those who are merely betrothed.