1 a folded paper container, usu. with a sealable flap, for a letter etc.
2 a wrapper or covering.
3 the structure within a balloon or airship containing the gas.
4 the outer metal or glass housing of a vacuum tube, electric light, etc.
5 Electr. a curve joining the successive peaks of a modulated wave.
6 Bot. any enveloping structure esp. the calyx or corolla (or both).
7 Math. a line or curve tangent to each line or curve of a given family.
Etymology: F enveloppe (as ENVELOP)
Useful english dictionary. 2012.