late 14c., "mark produced by pressure," also "image produced in the mind or emotions," from Old French impression "print, stamp; a pressing on the mind," from Latin impressionem (nominative impressio) "onset, attack," figuratively "perception," literally "a pressing into," from imprimere (see impress). Meaning "act or process of indenting" is early 15c.; that of "printing of a number of copies" is from 1570s. Meaning "belief, vague notion" (as in under the impression) is from 1610s.