The word seal has several meanings:

1. A sea mammal with flippers, whiskers, and sleek fur that lives in cold ocean waters. Seals are excellent swimmers who hunt fish underwater but come onto beaches or ice to rest and raise their pups. They can hold their breath for impressive lengths of time, some species diving deep for over an hour. You might see seals basking on rocks along the Pacific coast or performing tricks at an aquarium, balancing balls on their noses and clapping their flippers together.
2. To close something tightly so nothing can get in or out. You might seal an envelope by moistening the flap and pressing it down, or seal a jar of jam to keep it fresh. When you seal a leak in a tire, you plug the hole so air can't escape. The noun form means the closure itself: the seal on a new bottle of juice has to be broken before you can drink it. A hermetic seal closes so perfectly that not even air can pass through.
3. An official mark pressed into wax or embossed on paper to prove a document is authentic and important. Kings and governments used seals to show their authority, pressing their unique design into hot wax on letters and treaties. The President of the United States has an official seal showing an eagle, used on important documents. When you seal a deal or agreement, you make it official and binding.