n A paper or cardboard wrapper used to enclose small, flat items, especially letters, for mailing.
n Something that envelops; a wrapping
n A bag containing the lifting gas of a balloon or airship; fabric that encloses the gas-bags of an airship.
n (geometry) A mathematical curve, surface, or higher-dimensional object that is the tangent to a given family of lines, curves, surfaces, or higher-dimensional objects.
n (electronics) A curve that bounds another curve or set of curves, as the modulation envelope of an amplitude-modulated carrier wave in electronics.
n (music) The shape of a sound, which may be controlled by a synthesizer or sampler.
n (computing) The information used for routing an email that is transmitted with the email but not part of its contents.
n (biology) An enclosing structure or cover, such as a membrane.
n (engineering) The set of limitations within which a technological system can perform safely and effectively.