Most of us have a pretty good idea as to what constitutes overt cheating: sleeping with someone else, exchanging dirty pictures and text messages, sneaking out to meet with the person in private, and so on.
But then there are those "gray" areas where it can go either way: meeting up repeatedly for lunch, divulging personal details about each other's lives, constantly doing each other favors, buying each other things, etc.
If you find yourself in this boat and aren't sure whether your behavior can be construed as cheating, here's a rule of thumb:
If you'd feel uncomfortable telling your partner what you and the other person are doing, then it's probably cheating.
Just because the contact isn't physical doesn't mean there's no cheating involved. In fact, many people say they'd be more hurt and upset at finding out their partner is emotionally invested in someone else than discovering it's a purely sexual relationship.
People who cheat try to rationalize their behavior any which way they can -- from saying they're lonely to claiming the sex in their relationship or marriage has dried up. Instead of wanting the cake and eating it too, they should own up to their behavior and let their partner to decide whether they want to remain in the relationship or marriage.