Everyone defines "cheating" in their own way. Some strictly believe if there is no physical sexual contact it's not cheating.
Others would consider exchanging sexting photo or flirty sexual innuendo text messages as cheating.
There are two major components to cheating.
2. Lying directly or by omission to hide activities.
In order for whatever it is you do not to be considered cheating one's mate/spouse must be fully aware of what is going on and condones it.
Bottom line if you wouldn't do any sexting with your mate sitting right next to you odds are you know he or she wouldn't approve of it.
Most relationships begin with a certain amount of flirting, sexual innuendo, and so forth. If one continues engaging in such activities after they're supposedly in an "exclusive relationship" it appears as if they're still "shopping around" or "keeping their options open".
Such activities should be aimed at your mate to keep the magic alive.