1.NounA very simple design of key that usually has a cylindrical shaft (sometimes called a "shank") and a single, minimal flat, rectangular tooth or "bit".
2.NounA key that has parts filed away so that it will open a range of locks.
3.NounA master key.
4.NounA key (A crucial step or requirement) or (An idea, theory or observation that explains, justifies or renders (something) comprehensible).
"The skeleton key to this plan is the marketing, which will make the plan make sense when you see it."