polar adj.

"northern or southern end of Earth's axis," late 14c., from Old French pole or directly from Latin polus "end of an axis;" also "the sky, the heavens" (a sense sometimes used in English from 16c.), from Greek polos "pivot, axis of a sphere, the sky," from PIE *kwol- "turn round" (PIE *kw- becomes Greek p- before some vowels), from root *kwel- (1) "revolve, move round."

Originally principally in reference to the celestial sphere and the fixed points about which (by the revolution of the Earth) the stars appear to revolve; also sometimes of the terrestrial poles (poles of this world), the two points on the Earth's surface which mark the axis of rotation.

"having two poles;" see bi- "two" + polar. It is attested from 1810 in the figurative sense of "of double aspect;" by 1859 with reference to anatomy ("having two processes from opposite poles," of nerve cells). Psychiatric use in reference to what had been called manic-depressive psychosis is said to have begun 1957 with German psychiatrist Karl Leonhard. The term became popular early 1990s. Bipolar disorder was in DSM III (1980).