As a Noun:
1. An instrument, shaped like a knife, used for opening envelopes. English = letter opener.
2. A letter which has been put into an envelope, sealed and sent through the mail. It may already be opened, but usually has not yet been.
3. Letters that someone receives. English = correspondence.
As a Verb or Adjective:
1. To open a sealed envelope (usually with a letter opener) which contains a letter.