A Loop refers to a single Loop of time for a given universe. The amount of time that a Loop lasts can vary, though most universes tend to have a set time when Loops start and stop. (Bleach loops are an exception as they tend to last untill the death of the universe.)
Loops are usually published in compilations of short snippets. Each snippet tends to describe a single Loop, though it may reference other Loops, or contain a transition between one Loop to another. Snippets are anywhere from a few sentences to multiple paragraphs, depending on how in-depth an author decides to be.
A Loop is not considered to be canon unless it is included or planned to be in a published work. Even then, if it contradicts previously established Loops, it may be disregarded by some.
Canon of the looping story.