 The idea that there are scientific terms whose meaning is determined by a scientific theory goes back to Duhem and Poincaré. Such terms came to be referred to as theoretical terms in twentieth century philosophy of science. Properties and entities that are observable in the sense of direct, unaided perception did not seem to depend on scientific theories as forces, electrons and nucleotides did. Hence, philosophers of science and logicians set out to investigate the semantics of theoretical terms. Various formal accounts resulted from these investigations, among which the Ramsey sentence by Ramsey ([1929] 1931), Carnap’s notion of indirect interpretation (1939; 1958) and Lewis’s (1970) proposal of defining theoretical terms are the most prominent ones. Though not all philosophers of science understand the notion of a theoretical term in such way that semantic dependence upon a scientific theory is essential, this view prevails in the literature.