c. 1600, "kind of lobster," from sea + lion. Later the name of a fabulous animal (in heraldry, etc.), 1660s. Applied from 1690s to various species of large eared seals. As code name for the planned German invasion of Britain, it translates German Seelöwe, announced by Hitler July 1940, scrubbed October 1940.

1909 as a heraldic animal, 1964 as a U.S. proprietary name for brine shrimp (Artemia salina), which had been raised as food for aquarium fish but were marketed as pets by U.S. inventor Harold von Braunhut (1926-2003), who also invented "X-Ray Specs" and popularized pet hermit crabs. He began marketing them in comic book advertisements in 1960 as "Instant Life," and changed the name to Sea Monkeys in 1964, so called for their long tails.

lake of the River Jordan, mid-13c., from dead (adj.); its water is 26 percent salt (as opposed to 3 or 4 percent in most oceans) and supports practically no life. In the Bible it was the "Salt Sea" (Hebrew yam hammelah), also "Sea of the Plain" and "East Sea." In Arabic it is al-bahr al-mayyit "Dead Sea." The ancient Greeks knew it as he Thalassa asphaltites "the Asphaltite Sea." Latin Mare Mortum, Greek he nekra thalassa (both "The Dead Sea") referred to the sea at the northern boundaries of Europe, the Arctic Ocean.

"huge, hideous, or terrible marine animal," 1580s, from sea + monster. Sea serpent is attested from 1640s. In Middle English a sea-monster might be called sea-wolf; in Old English, sædraca "sea dragon," or sædeor "sea-animal."

"commander of an ocean-going vessel," 1610s; see sea + captain (n.).

in geopolitics, "nation having international power or influence at sea," by 1849, from sea + power (n.).

1670s, "venomous sea-snake of the tropics," from sea + serpent. By 1774 as "enormous marine animal of serpent form," figuring in mariners' tales.

1796, echellon, "step-like arrangement of troops," from French échelon "level, echelon," literally "rung of a ladder," from Old French eschelon, from eschiele "ladder," from Late Latin scala "stair, slope," from Latin scalae (plural) "ladder, steps," from PIE *skand- "to spring, leap" (see scan (v.)). Sense of "level, subdivision" is from World War I.

"fireman's scaling ladder," short for pompier ladder (by 1893), French, literally "fireman," from pompe "pump" (see pump (n.1)).

1650s, "resembling a ladder," from Latin scalaris "of or pertaining to a ladder," from scalae (plural) "ladder, steps, flight of steps" (see scale (n.2)). The noun in the mathematical sense of "a real number" is from 1846, coined by Irish mathematician William R. Hamilton (1805-1865), who can explain why it is the correct word for that.