deep-sea adjective 3deep sea noun 2forms 1 adjective3 •Of, related to or located in the deeper part of the sea.The oarfish is a deep-sea fish. •Of or related to an open sea. •Of a port, able to accommodate the largest ships. deep sea noun2 •The deeper part of the sea or ocean in which no light penetrates. •An open sea. (the part of the sea that is out of sight of land) formsdeep seas