What is a theoretical term? This question can be answered in at least two different ways. First, a theoretical term is simply a non-observational term. Second, a theoretical term is one whose meaning depends on the axioms of a scientific theory. According to the first explanation, a theoretical term cannot be applied using just unaided perception, without drawing inferences. This explanation defines the notion of theoreticity merely as the absence of observability. The second explanation, by contrast, has the virtue of giving a positive characterization of the notion of theoreticity. Both explanations stand in the need of further elaboration. If we characterize theoretical terms by non-observability, we need to explain what an observational term is. There is no consensus in the literature as to whether and, if so, to what extent it is feasible to draw the theory-observation distinction. On the one hand, critics of the theory-observation distinction have often attacked only weak proposals of how to draw the distinction in question. On the other hand, the extreme skepticism by Thomas S. Kuhn, Paul K. Feyerabend, and Norwood R. Hanson concerning the distinction is increasingly losing consensus among contemporary philosophers of science. This is evidenced, for example, by attempts at exploiting the formal semantics of theoretical terms in one version of structural realism. If we explain the notion of a theoretical term by way of semantic dependency upon a scientific theory, we need to give an account of this semantic relation. How does a theory determine the meaning of a theoretical term? What, if any, are the differences between theoretical terms and defined terms? How can we distinguish, in a sensible way, between the synthetic assertions of a scientific theory about the world and meaning postulates determining the meaning of theoretical terms? Various formal semantics of theoretical terms have been devised in order to answer these questions. Notably, the idea that the meaning of a theoretical term is determined by a scientific theory, or a set of such theories, has already been expressed by Pierre Duhem and Henrie Poincaré. The theory-observation distinction can be applied to syntactic and semantic entities. Thus, we can speak of theoretical terms and theoretical concepts. Moreover, we can speak of theoretical entities, in the sense of specific objects that are the referents of theoretical concepts. Philosophical research on theoreticity concerns syntactic aspects inasmuch as semantic aspects of theoreticity.