Sea  n. 1 expanse of salt water that covers most of the earth's surface. 2 any part of this. 3 named tract of this partly or wholly enclosed by land (north sea). 4 large inland lake (sea of galilee). 5 waves of the sea; their motion or state (choppy sea). 6 (foll. By of) vast quantity or expanse. 7 (attrib.) Living or used in, on, or near the sea (often prefixed to the name of a marine animal, plant, etc., having a superficial resemblance to what it is named after) (sea lettuce).  at sea 1 in a ship on the sea. 2 perplexed, confused. By sea in a ship or ships. Go to sea become a sailor. On the sea 1 = at sea 1. 2 on the coast. [old english]