"traditional literary theme," 1948, from Greek topos , literally "place, region, space," also "subject of a speech," a word of uncertain origin. "The broad semantic range renders etymologizing difficult" [Beekes].

"a radioactive isotope," 1946, from radio- , combining form of radiation , + isotope .

before vowels often is- , word-forming element meaning "equal, similar, identical; isometric," from Greek isos "equal to, the same as; equally divided; fair, impartial (of persons); even, level (of ground)," as in isometor "like one's mother." In English used properly only with words of Greek origin; the Latin equivalent is equi- (see equi- ).