Proto-Indo-European root meaning "body of water."
It forms all or part of: aquamarine; Armorica; beche-de-mer; cormorant; mare (n.2) "broad, dark areas of the moon;" marina; marinate; marine; mariner; maritime; marsh; mere (n.1) "lake, pool;" Merlin; mermaid; merman; meerschaum; meerkat; morass; Muriel; rosemary; submarine; ultramarine; Weimar.
It is the hypothetical source of/evidence for its existence is provided by: Latin mare; Old Church Slavonic morje, Russian more, Lithuanian marės, Old Irish muir, Welsh mor "sea;" Old English mere "sea, ocean; lake, pool," German Meer "sea."