An outer casing produced by many [[larva|larval]] [[bug|insect]]s, such as [[caterpillar]]s, for the purpose of metamorphosis into adulthood.

Can also refer to things that are completely covered and bound for other reasons, such as prey caught by a [[spider]].

Frequently used in fiction as a means to achieve [[mutant|mutation]]s, [[evolution]], or other [[growth]]s and [[transformation]]s.