J.-
yes, it is cheating. people will define it in different ways, but whether it was physical or emotional, he crossed a line for sure. its kind of difficult to give you advice because i don't know you or your husband. i think if i did, it would make things a lot easier because this is such a personal issue - but i will try! :)
i read an interesting article in a magazine this month about "why men cheat" and it brought about some points that may be something for you to think about. it said that less than 8% of men just cheat for sex - that 50% know the woman for about a month before something happens or they cross that line - that about 60% cheat for emotional reasons (not feeling appreciated at home, not feeling important, they miss their wife, bad communication between he and his wife, etc.) i am NOT saying that this is your fault in ANY WAY! but perhaps there is a part of your marriage that needs some attention from you BOTH, and perhaps if you two can spend some time focusing on that, you can set aside this incident and heal your marriage together. i liked the statistics in the article, but the overall read made it feel as if it was the woman's job to make things better - i disagree with that - it needs to be BOTH working on it equally, and the husband doing what he can to regain that trust! that, i do believe!
it sounds like your husband and this woman are no longer in contact, and that he has moved away from it. i understand that you must be so hurt and angry and confused right now - no doubt about that. once you think you can, my advice would be to sit down and really talk - i mean really talk (not yell and argue, if possible - no matter how right you are in being mad and hurt and pissed and how justified it would be to come unglued on him) - but try to get issues and feelings out there and make a plan for how you two will have a better tomorrow. try to use "i" statements - ("i feel very hurt and i want for us to be able to be honest with each other and really work on this." - instead of finger pointing and blaming and ugly words) it will be a far more productive conversation if you are both careful about HOW you talk to one another.
good luck - this is a hard one, and i am sorry you have to deal with it. i am sure you'll get some great advice here... maybe from someone who has "been there." do what your heart tells you, follow what's right for you, and you'll find your way. i'll be thinking of you.