means Water containing dissolved salt, especially the kind that fills the oceans and seas.

from A plain compound of two ancient English words. 'Salt' descends from Old English 'sealt', part of a vast family of Germanic and Latin salt-words (Latin 'sal', source of 'salary' — Roman soldiers were said to be paid in or for salt). 'Water' comes from Old English 'wæter', a cousin of German 'Wasser' and rooted in the same Proto-Indo-European source that gave Greek 'hydor'. Bolted together, the word does exactly what it says — salt plus water — and has done so for as long as English has needed to tell the sea from the river.