Open English Wordnet
Nouns
(n)envelopea flat (usually rectangular) container for a letter, thin package, etc. (n)envelopea curve that is tangent to each of a family of curves (n)envelopea natural covering (as by a fluid)“the spacecraft detected an envelope of gas around the comet” (n)envelopethe maximum operating capability of a system (especially an aircraft)“test pilots try to push the envelope”