What is emotional cheating, and where is the line?
Investing emotional intimacy in someone outside the relationship in a way that would not survive being disclosed — which is the most workable definition available, because there is no behavioural checklist that fits every couple.
Why no fixed line exists. Couples differ enormously and legitimately in what they regard as private, as flirtation, or as a threat. A definition based on specific acts fails immediately: the same lunch means different things in different relationships. A definition based on concealment and comparison travels much better.
The markers people consistently identify:
Secrecy. Deleting messages, not mentioning the person, describing the friendship differently to different people. This is the strongest single signal, because concealment implies an internal judgement has already been made.
Diverted disclosure. Significant news, worries and small daily details going to the other person first — since disclosure is the mechanism by which intimacy is built, redirecting it redirects the intimacy.
Comparison. Using the other person as a benchmark the partner is measured against.
Withdrawal. Energy, attention and patience reduced at home in proportion to what is invested elsewhere.
Escalating privacy. Moving to channels the partner would not see.
What it is not. Having close friends, finding other people attractive, having a rich inner life, or maintaining friendships with people one could theoretically be attracted to. A relationship requiring the surrender of all other closeness is not a healthier relationship, and framing ordinary friendship as betrayal is itself a controlling pattern worth naming.
Why it hurts as much as it does. Research on what people find most threatening finds emotional involvement frequently rated worse than a purely physical encounter, because it implies the attachment itself was relocated, and because trust in the ordinary account of someone's life is what gets broken.
The practical test worth applying: would you be comfortable if your partner read this exchange, and if they were sitting here? Discomfort is the information.