Seal

A wax or other item used to close an item to indicate whether or not the item has been opened since the seal was applied.

A letter writer would compose the letter, fold it over, pour wax over the joint formed by the top of the page of paper, and then impress a ring or other symbol to:

• Seal the document to prevent tampering
• Provide proof that the document is actually from the sender and is not a forgery

Also, a marine mammal; a pinniped.