legal wife; lawful wife
Refers to a wife recognized by law or formal marriage, as opposed to a mistress or common-law partner.
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.
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.