noun
[ U ] the average height of the sea / ocean, used as the basis for measuring the height of all places on land :
50 metres above sea level
noun
[ U ] the average height of the sea / ocean, used as the basis for measuring the height of all places on land :
50 metres above sea level
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005