One level of a thing built in levels, each resting on the one below. Layers can be reasoned about separately, which is their whole use.
Old English lecgan (to lay, place), causative of licgan (to lie); a 17th-century agent formation for one level of a thing built in resting levels.
A level that can be reasoned about on its own, which is the entire reason for building a thing in layers rather than all at once.