Kept woman and Significant other

Kept woman noun - An adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man.

Significant other and kept woman are semantically related. In some cases you can use "Significant other" instead a noun phrase "Kept woman".

Significant other noun - The person to whom another is married.

Kept woman and significant other are semantically related. Sometimes you can use "Kept woman" instead a noun phrase "Significant other".