a mistress who is kept after one becomes older
老後になって持つ妾 - EDR日英対訳辞書
a keptmistress who is housed in a residence other than the home of the man with whom she is involved
本宅以外に囲っておく妾 - EDR日英対訳辞書
Although the nyobo feared the consequences and kept the incident a secret, Ryuju no mae (龍樹の前), who was a Kagesue's mistress, disclosed the secret and Kagesue told Kagetoki about it.
女房所は後難を恐れてこれを秘匿していたが、景季の妾の龍樹の前がこれを語り、景季が景時へ伝えた。 - Wikipedia日英京都関連文書対訳コーパス