What would People class as cheating?

I’m just wondering what people’s thoughts are on what cheating actually involves? Me and my other half have been a bit rocky lately we had a miscarriage in September we split up in October but we did get back together a few days later, I found in that time he had been talking to another girl arranging to meet her etc all the whole ignoring my messages to try again, anyways we did try again and got back to normal but he had a night out with the boys and didn’t come home, as bad as it sounds I had a gut feeling and had a look on his phone he had been talking to another girl on this night out she even sent him naked selfies I didn’t get to see his replies I just seen the screen shot of her selfies, I have sat on this for a week now and not mentioned it too him as we were trying to convince again but I don’t no what to do for the best anymore