letter opener

Short definition from Wiktionary

Noun

1. 1
   Any of several devices for opening letters, either by breaking wax seals or (as is usually the case today) by cutting open their envelopes: A device shaped like a fixed-blade knife, usually with a blunt edge; the blunt edge does not slice the envelope so much as precisely tear it.
2. 2
   Any of several devices for opening letters, either by breaking wax seals or (as is usually the case today) by cutting open their envelopes: The same sort of device but with a sharpened edge, designed to appeal to customers who enjoy a sharpened one (regardless of whether or not it truly outperforms the nonsharp counterpart).
3. 3
   Any of several devices for opening letters, either by breaking wax seals or (as is usually the case today) by cutting open their envelopes: A device incorporating a small razor-style blade in the crotch of a U-shaped plastic handle; it slices the envelope while being, like a safety razor, unlikely to cut the user accidentally.