I don't associate with cheaters, and I won't date a cheater that doesn't have a thoroughly valid excuse for it (abuse, etc).

Would you ever excuse it?

- No

What do you consider cheating? 

- Getting physically or emotionally(romantically) involved with a person while having a formal relationship with another one. 

Cheating shouldn't be an issue if you are in a healthy relationship in the first place. (And this applies to open relationships as well, it's all about trust.)
Cheating is usually born out of dissatisfaction and feelings of inadequacy on the part of the cheater, so it's more of a symptom of an underlying problem.
Basically, one, or both people in the relationship weren't ready for it and that's why it didin't work out.

Usually people don't reconize the warning signs and aren't comfortable to be open and honest enough about their own feelings, either because they hide them from themselves or because they don't want to hurt the other person and that lack of communication makes the relationship unstable and eventually unsustainbale.

The chances are, if they cheated once, they'll do it again.

Be honest with your partner if you want an open relationship or if you are no longer happy. Misleading one or more people in a relationship is dishonest and ends with someone getting hurt.

A relationship is built on trust, so trying to cover it up isn't the best way to handle this situation. As you said you don't want her to find our through someone else. So I suggest you grow a pair and confront her with the situation and evaluate your current relationship after.

IMO what you did was stupid and not very chivalrous since you are in a relationship. On the other hand you regret it from happening and don't really have emotional ties with the other person. So if it were my relationship I wouldn't deem the act unforgivable, however keeping quiet and trying to shove it under a rug would.

Anyway I would say she deserves to know. As for the question if it's cheating. Yes but a relatively light grade, but it was still cheating.

from a personal standpoint, i have never cheated. if i like somebody i stay committed to them, and if i feel like i dont like them anymore or get feeling for somebody else, i call it off before anything happens.

but if youre looking for some solid advice for your situation, i would say you gotta come clean and admit to her what happened. she will take it best coming from you rather than somebody else.

If she kissed you, you should have told your girlfriend and try to solve this out. Everyday that passes by, it becomes a bigger thing and more hard to forgive. The thing is: how would you feel if it was the other way around? Not trying to make you feel guilty, because that's with you and different people have different thoughts about what is cheating, but I think you should wonder what would you want your girlfriend to do if she betrayed you. And then do it. Seriously, it's very hard to live with a secret like that, always afraid of being discovered. 

I never cheated, at least not physically, but in the past I had strong feelings about a girl while being with my ex-GF. Eventually, we broke up and I forced myself to wait a few months to be with the girl that I liked, because I didn't want to feel as I was cheating on my ex or exchanging her. 

I'd never cheat but I have been cheated on. So yeah I'm against it

Stuff happens would have been worse if you actually had sex with the other woman.