Sleep systems are the combinations of cover and insulation that are sufficient to keep you warm and dry while sleeping. Sleep systems consist of a shelter such as a tent or tarp, sleeping bag or quilt and usually a sleeping pad. Hammockers may not use a sleeping pad and instead may opt for an under quilt to provide insulation under their hammock.
A sleep system can also be a general term for a sleeping bag or quilt, when it refers to the insulation and the cover that is appropriate for a given environment. Used in discussions to define the opposing factors of adequate warmth for the environment versus carrying weight.