Cheating is a broad term (and a serious term) in the lifestyle. Many swingers are not bound to society's definition of cheating due to the communication and extenuating circumstances in any given relationship. Each grouping of people set their own rules, their own bounds and deviating from that is cheating.
Cheating, also known as exploitation, between organisms is a form of parasitism or specialized predation in which an organism engages in what appears to be a mutualistic relationship with another organism, but does not in fact provide any benefit to the other organism. The relationship between the organisms may be symbiotic, in which case cheating is a form of parasitism, or non-symbiotic, in which case cheating is a form of predation or herbivory.