The 12 recurring signs of an emotional affair, the 7 stages emotional affairs move through, and how to tell an emotional affair apart from a normal friendship.

An emotional affair is a sustained non-physical relationship with someone outside the primary partnership that involves emotional intimacy, secrecy, and a level of attention or priority that would normally belong to the primary partner. The physical line does not have to be crossed for the affair to be doing real damage — the exclusivity line is the one that gets broken.
The recurring signs are: talking about one specific person constantly then suddenly stopping, a changed phone posture (face-down, taken to the bathroom, angled away), defending the outside person more than the relationship, sharing news with the outside person first, emotional withdrawal from the primary partner, dressing differently for that person, long conversations labeled as 'just venting,' and small gifts or inside jokes the partner does not know about.
Innocent connection, preference forming, private channel opens (moving from group to one-to-one), emotional disclosure, secrecy, emotional exclusivity (the outside person is now the first call), and finally rationalization or physical crossing. Naming the stage makes it easier to see which one is currently happening.
A friendship is visible (the partner knows about it), bounded (intimate disclosures still go to the partner first), and survives disclosure (the messages would not need to be edited before the partner sees them). An emotional affair is engineered around the partner's absence and would not survive a line-by-line read of the messages.
Yes, when three conditions are met: full contact ended with the outside person, full disclosure of the timeline (not a drip-feed), and a sustained period of restored transparency long enough to matter (typically six to twelve months). Missing any of the three tends to end the same way it started.
In long-term surveys and clinical work (Shirley Glass's research is the standard reference), emotional affairs are consistently reported by betrayed partners as more damaging than short-term physical affairs. The reason is structural: an emotional affair replaces the partner instead of supplementing them, which is harder to recover from than a one-off physical breach.