This paper aims at offering an all-encompassing definition of metonymy. First, metaphor and metonymy are compared in order to see what are the main features that characterize these two cognitive processes. In doing so, some of the previous work carried out is critically reviewed. On the basis of the analysis, the author observes that the only distinguishing feature between them is the domain-internal/domain-external nature of the mapping. Furthermore, he argues that there exists only two basic types of metonymic mappings regarding the nature of the domains involved: target-in-source and source-in-target. Moreover, he describes metonymy as a one-correspondence mapping in contrast to metaphor which can be either a one-correspondence or a many-correspondence mapping. Finally he explores the communicative effects of metonymy and examines in detail the main problems that the Relevance Theory approach poses. (Francisco Ruiz de Mendoza and Diez)