seal

• seal [countable]
• 1 a large sea animal that eats fish and lives around coasts
• 2
• a) a mark that has a special design and shows the legal or official authority of a person or organization:
• The document carried the seal of the governor's office.
• b) the object that is used to make this mark
• 3 a piece of rubber or plastic that keeps air, water, dirt etc out of something
• airtight/watertight seal
• an airtight seal around the windows
• 4 a piece of wax, paper, wire etc that you have to break in order to open a container, document etc
• 5

   seal of approval

  if you give something your seal of approval, you say that you approve of it, especially officially:
• A number of employers have already given their seal of approval to the scheme.
• 6

   set the seal on something

  British English to make something definite or complete:
• In 1972, Nixon himself went to China to set the seal on the new relationship.

Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.