The term 'OPEN AN ENVELOPE' can be solved by two crossword solutions: UNSEAL (6 letters) and SLIT (4 letters). Unsealing an envelope involves carefully removing the adhesive seal that is used to keep the envelope closed. Slitting an envelope involves cutting along the top of the envelope to open it. Both methods are commonly used to open envelopes, although unsealing is generally preferred as it does not damage the envelope.