An Australian psychologist says 'micro-cheating' is a thing and is happening a lot more these days. 

The loose definition is this: A subtle betrayal involving secrecy.

Example from the article: 'You might be engaging in micro-cheating if you secretly connect with another person on social media, if you share private jokes, if you downplay the seriousness of your relationship to your partner or have a code name in your phone for them.'

HERE'S THE FULL ARTICLE