Affair

An affair is when two people who are not supposed to be together in that way, like if they are both married or in a relationship, do things together that are meant to be only done with people you are in a romantic relationship with, like kissing or being intimate with each other. This is usually wrong because it breaks the trust of the people they are in relationships with and can hurt their feelings very much. It's like when you play with a toy that belongs to your friend without asking – it's not okay because it's not yours, and your friend might feel sad.