Any kind of constructive, non-platonic contact with another person that's not discussed and expressly agreed upon in advance and conducted in private/away from the other spouse/SO (expressly for the purpose of concealing the activity), or goes beyond the parameters agreed upon.

In my opinion, you prefer to be with them or kiss them, you might as well fuck them. I have a no tolerance policy on this, sorry.

I don't think this really applies to me, but I thought I'd throw down my two cents anyway (I'm a queer -read "other than straight" woman, and my "other half" is a straight man) The question doesn't really apply because we are an intentionally non-monogamous couple. My answer from our perspective, though, would be that cheating would be occurring if there was any lying, game-playing, or disrespect of the agreed upon boundaries of our relationship: I would not like for him to have unprotected sex with someone else, nor would I do that. I would not want him to feel uncomfortable telling me about someone he was seeing, and if I felt uncomfortable telling him, I'd take a good, hard look at why. We answer direct questions directly. If we can't, there's a problem. We've been doing this for about a year, and so far it's worked out fairly well. Sometimes one of us feels a little left out when one of us has a new playmate, but all in all, I don't think I'd ever want a relationship that doesn't work this way --with him or with anyone else. We've tried sharing a female lover, but it worked out better for him than me, sex-wise (she was "bi-curious" and decided after about four minutes that, no, really she was straight). He still sees her on occasion, and she and I go to brunch sometimes. She feels somewhat awkward about it at times, but... that's her stuff, not mine, not ours.
Maybe I have an unconventional view on cheating, but as I've said to people in the past who were in relationships with others and wanted to see me anyways: If you feel weird or anxious about it, if it falls outside of the agreed-upon boundaries of your relationship, or if you think your partner wouldn't approve, it's probably cheating.

we are non-monogamous, hope-to-be polyamorous couple. if i do something we a person we haven't agreed is "safe," then that's cheating. we have to both agree and i need the green light.

If it wasn't agreed upon before the act took place then it's cheating in my opinion.

It kind of depends. While I consider any kind of kissing, sexual stuff and even emotion cheating as cheating, for me there is a difference in misconduct and something that I would break up with my lover for. If he lies about it, then it is break up worthy. So is sexual/physical cheating.

My boyfriend said he would be okay with me kissing other girls, but I wouldn't be okay with it. He might not consider it cheating, but I do.

Cuddling, dancing, flirting, and light kissing are always ok to do with anyone. Making out, grinding, neck biting, and the like are also okay to do, but it's better to limit that sort of thing to just with friends. Stuff that can be described using the term "sex" - everything from cybersex/phone sex to manual sex to oral sex to intercourse - can only be done between people in our triad, unless we specifically negotiate and discuss it beforehand. I'm in a polyamorous relationship, though, so the limits are obviously different than many people's.

We have "rules" for our relations in this manner and they have been developed over time, lots of talking, and figuring out what works and what doesn't. I'm bi and he's straight, and I have been with him the entire time I've been figuring out my bi side. Mainly, we both have to want to be with the 3rd female. She has to understand our relationship and respect it. After being together for a while, we can play seperately, but we keep each other in the loop when that's going on too. Every couple is different, you just have to figure out what works for you.

My boyfriend considers just about everything to be cheating. Cuddling, hugs that are too intimate, any kisses other than on the cheek, any form of sex. Massages are cheating, flirting in excess may be considered cheating but I'm naturally a flirt so a little bit of flirting (like when I don't realize I'm doing it) is alright. These rules apply for everyone, not just one sex or the other (and he doesn't quite understand the difference between gender and sex)