JACA, a city of Spain, in the province of Aragon. It is situated at the foot of the Pyrenees, near the river Aragon, from which the province has derived its name. The situation is on a plain of fertile land, between two lofty sierras. There are in it considerable manufactures of cloths and baizes. It is a place of great antiquity, and of considerable strength; the walls, of ancient date, are flanked with towers, and the citadel attached to it is capable of considerable resistance. It contains 4000 inhabitants, and is in longitude 0. 41. W. and latitude 42. 29. N.