Envelope: countable noun. A flat container, usually made of paper or plastic, often sealed with glue or wax, used to mail letters or small objects. Jack put fifty cents in an envelope and placed it beside his son's bed in exchange for the tooth under the pillow.
Envelop: transitive verb. To surround completely; to overwhelm. Maria's favourite time of day was dusk, as she waited for the darkness to envelop the neighbourhood.