The sea level is the average height of the ocean (informally called the sea). The word 'average' must be used because the height of the sea changes with the tides.

The height of mountains, countries, and so on, is almost always given as "above sea level".

Geography

Meeresspiegel | Sea level | NAP | גובה פני הים | Livello del mare | poziom morza | Zeeniveau | Nível do mar | Havsyta | 海平面