A seal is any of several different species of marine mammal of the suborder Pinnipeda. The seals commonly so-called are in the families Phocidae (the earless seals) and Otaridae (fur seals and sea lions).
A seal is a wax or other item used to close a letter, parcel, or other item, to indicate whether or not the item has been opened since the seal was applied. Seals are no longer commonly used. Seals were used both to seal the item to prevent tampering, as well as to provide proof that the item was actually from the sender and is not a forgery. To seal a letter, for example, 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, metal stamp, or other device.
Seal is a musician from the U.S. who was popular for a while in the 1990s.