What do you consider an emotional affair?

Generally speaking, I would consider it one in which a person already entered into a commited relationship of some sort witholds part of themselves from that relationship and turns it elsewhere. Strangely enough, I don't think this necessarily needs another person.

For example: fantasies. Your significant other, if they aren't sharing their fantasies, are in a way having an affair with those fantasies. You are being emotionally blocked off from some aspect of them.

This could also mean partner/s. Your significant other could be more mentally/emotionally intimate with other people than with you.

Is this cheating? Not in my opinion. I think everyone has their secrets and your mind is the only thing that will ever truly belong 100% to you. How you chose to spread it around is up to you.