late 14c., "a quarter of a day, six hours," from Middle French quadrant, from Latin quadrantem (nominative quadrans) "fourth part," also the name of a coin worth a quarter of an as, noun use of present participle of quadrare "to make square; put in order, arrange, complete; run parallel, be exact," figuratively "to fit, suit, be proper," related to quadrus "a square," quattuor "four" (see four). The surveying instrument is first so called c.1400, because it forms a quarter circle. Related: Quadrantal.