Is it considered adultery if you have sex with someone other than your spouse while separated?

My husband has a girlfriend that he has had stay over (had sex with) introduced her to his father, sister, and our two boys who are 20&21 within two weeks of asking me to leave and before even filing for separation. The reason for our separation is because I accused him of seeing someone else. He says he did nothing wrong because we were not together. The exact same thing happened a year and a half ago. Is this still considered adultery?