I reckon kissing is cheating. You don't usually kiss anybody else apart from your bf/gf at the time, that's why such a big deal is made over the "first kiss" of a new relationship. Yep, kissing is definitely cheating.
I'd say cheating was doing anything with a third party that would harm the relationship between you and your partner. So it's relative to how they'd see it; some couples may not class a kiss as cheating, others may.
You can define many things as cheating even a simple hug but it all depends on the context of it and for example if its a kiss what kind of kiss a peck on the cheek or a full game of tonsel hockey!
is it relative on how far you have gone with a partner eg if you had slept with them then its ok if they kiss someone else>? or is kissing cheating?
Depends on what sort of kiss it was, If it was just a friendly kiss then its harmless, but if it was more than just a friendly peck then it is cheating.