Historical term for a secondary wife or concubine of a noble, especially in pre-modern Japan. Contrasts with 正室 (せいしつ), the legal wife.
See also: 正室
In the Edo period, daimyo often had concubines in addition to their legal wife.
側室 means a lord's concubine, not an aide; learners may confuse it with 側近 because both start with the kanji 側.
Compound of 側 (そく, 'side') and 室 (しつ, 'room' or 'wife'), literally meaning 'side room' or 'secondary wife'.