Halachic Definition of Adultery

Shalom aleichem Rabbi Lauffer. Thank you for answering my last question. In “Thursday Nights with Rabbi Avigdor Miller Vol. 1” on page 166 Rabbi Miller says “I know a case, in Williamsburg, where a woman made a very big mistake going to a chiropractor. He was somewhat free with her, but she said to the rabbonim nothing happened. The rabbonim discovered that there was something that would seem nothing to her, but in the halacha it was more than nothing. It was something serious. She had to take a divorce from her husband…It’s a tragedy, but it can’t be helped. But once she committed adultery, it was over-and adultery doesn’t mean what you think adultery means, even if it may seem quite innocent. It’s the same as adultery.” What, besides actual intercourse, constitutes adultery according to the halacha? Thanks a lot.