Simple and clear at the first glance, the word sea, having Indo-European parallels which have practically the only meaning ‘sea’, on a closer examination turned out to be not so clear. Linguistic and ethno-linguistic, including folklore, data of Slavic traditions make it possible to interpret the word sea in Slavic languages and dialects in the sense of ‘water’ in general, and especially – the vast expanse of water – regardless of salinity, flowage and other properties of water. Moreover, the same semantics are characteristic of many, if not all, Indo-European denominations of the sea, and many words of non-Indoeuropean languages as well. Aspects of future studying of this topic can be seen in the explanation of other words with root *mor-/mar-, identification of their semantics, motivational relations and etymology, as well as other lexems, denoting the sea, the causes of their emergence or preservation in i.-e. languages and solving a number of other problems of diachronic aspect.