Emotional infidelity is developing a close, intimate bond with someone outside your relationship, the kind of bond that would usually threaten your relationship if it's revealed. For instance, talking to your co-worker all the time, revealing intimacies about your marriage or your relationship, having a very deep, meaningful connection that would threaten your relationship would be considered emotional infidelity.
Emotional infidelity usually starts out very innocently. People start talking, they start sharing confidences and before you know it that relationship becomes so strong that often it interferes with your primary relationship. A lot of times people involved in emotional cheating will divulge secrets of the primary relationship, will often complain about primary relationship and will spend more time in that emotional relationship than in their primary one. Over time it is extremely destructive to the primary relationship because the confidences that should be shared, the emotions that should be shared in the primary relationships are shared outside with someone else.
If you feel that you are developing an extremely strong emotional bond with someone outside your relationship it is very important to share that with your partner. The first step would be to disclose how close you feel to someone else to your partner and to attempt to develop a greater closeness with your partner. Of course another important step is to slowly become less involved emotionally with the person outside of your relationship.