verb (envelops, enveloping, enveloped)[with object]
late Middle English (formerly also as invelop(e)): from Old French envoluper, from en- 'in' + a second element (also found in develop) of unknown origin
Envelop is a verb, stressed on the second syllable and meaning ‘wrap completely’. The noun meaning ‘paper container for a letter’ is envelope, stressed on the first syllable.
Do not confuse envelop with envelope. Envelop means 'cover or surround something completely' (the mist seemed to envelop the boat), whereas envelope means 'a paper container for a letter' (a stamped addressed envelope).