How does the author define an "affair" in the nonfiction book, After the Affair?

A marital affair is typically defined as sexual intercourse between a spouse and a non-marital partner. The text suggests that this definition, while usually correct, is not broad enough to include all aspects of all affairs. The text instead defines an affair as a serious breach of trust that results in intense feelings of betrayal. For example, an affair thus may encompass intimate behavior not involving sexual intercourse.