Cheating like beauty is in the eye of the beholder.
For me personally cheating is usually connected with lying either directly or by omission. It's any act that disrespects the relationship which could include flirting, using sexual innuendo, secretly spending time with another knowing full well your mate would not approve of it, kissing, holding hands, and naturally having sex of any kind. This would also include "cyber sex" via webcam with someone your mate has never met in the flesh.
There is also a such thing as "emotional cheating" where one pours their soul out to another spilling details of hopes, dreams, and wishes they have never shared with their "significant other". This type of act forges the type of intimacy that should be reserved for your mate.
One of the best things a couple can do is have an open and honest conversation about what constitutes cheating (early on) their relationship. This will give each other an idea of whether or not they are on the same page. Having different expectations of each other can lead heartbreak.