What constitutes cheating?

What constitutes cheating?

In my opinion, anything that the partner wouldn't consent to if they knew, and you have to hide it from.

Might sound ironic, but for example my wife and I do (very) occasionally involve another woman for special activities, and this is not cheating in our book. But if I confided in another woman for example, more than I did with my wife, without her knowing, that would be cheating for her, even if nothing phisical happened.

What constitutes cheating?
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