means The act of making something smaller, fewer, or less — in amount, size, or complexity; in cooking, the process of simmering a liquid down until it thickens and intensifies.

from From Latin reducere, "to lead back" — from re- ("back") plus ducere ("to lead," the same root that gives us conduct, duke, and aqueduct). The earliest English sense was literally about leading or bringing something back to a former state; only later did it slide toward the modern meaning of cutting down and boiling away, until the word itself became a kind of reduction of its older self.