What is considered cheating?

So for the last several months something has felt "off" with my husband and I. Recently there have been numerous red flags so I finally started to check some stuff out. Turns out since the beginning of the year he has been messaging an ex of his. She knows he's married and he even talks about me. He's been asking for naked photos and when she told him no he said he would just go back through his old ones of her bc she will always be his little porn star. He told her he made a mistake marrying me and should have stayed with her and that he has always loved her. They have talked several times but it's mostly messages on fb. He makes reference to accidentally deleting her number when he was trying to delete their conversation. They haven't had sex from what I can tell but it still feels like cheating to me.