legal wife; lawful wife
Refers to a wife recognized by law or formal marriage, as opposed to a mistress or common-law partner.
彼は正妻のほかに愛人がいるらしい。
He seems to have a mistress besides his legal wife.
Used in contexts of polygamy to distinguish the primary wife from secondary wives or concubines.
歴史ドラマでは、正妻と側室の争いがよく描かれる。
Historical dramas often depict conflicts between the first wife and concubines.
正妻 is the legal wife, the opposite of 裏方 in its old sense of a concubine or secondary consort.
Compound of 正 (sei, 'correct, legitimate') and 妻 (sai, 'wife'). The reading せいさい is the on'yomi of both characters.