The paper deals with some logical, semantic and methodological aspects of defining and definitions. First of all, basic features of the background semantic theory are specified. Next, three different kinds of definitions are distinguished: codifying definitions, objectual analytic definitions and meaning analytic definitions. It is shown that the relationships between the definiendum and the definiens in a given definition differ with respect to these different kinds of definition. Furthermore, it is argued that definitions and defining are intrinsically connected with so called conceptual systems. Finally, definitions are distinguished from explications.