legal wife; lawful wife
Refers to a man's legally recognized wife, often in contrast to a mistress or concubine. The term carries a formal, somewhat traditional tone.
彼は本妻の他に愛人がいるらしい。
He seems to have a mistress in addition to his legal wife.
本妻の座を守るために彼女は努力した。
She worked hard to maintain her position as the legal wife.
Compound of 本 (ほん, 'main' or 'true') and 妻 (さい, 'wife'). The reading さい for 妻 is the on'yomi, commonly used in compounds.