A country is landlocked if it has no access to any oceans or seas. As an example, Austria is landlocked. France is not landlocked because it has access to the Mediterranean and the Atlantic Ocean. A sea is also landlocked if does not have access to an ocean. This is also called an endorheic basin. As an example, the Caspian Sea is landlocked. The Gulf of Mexico is not landlocked because it has access to the Atlantic Ocean.
Examples of landlocked countries:
If a country is landlocked by landlocked countries (surrounded by landlocked countries), then it is doubly landlocked. Liechtenstein and Uzbekistan are the only two doubly landlocked countries in the world.