Traditionally, children’s definitions of a given noun have been considered as correct only if they fitted the classic Aristotelian model of definition: “X is a Y that Z” (see Snow, 1990). In a different perspective, the aim of this study was to identify the different types of definition children use, in order to define different domains of knowledge in the context of their developmental trend. Thus, the definitions provided by 10, 11and 13-year-old children for concept nouns referring to different knowledge domains were collected and coded according to their different types. The results showed that, while children at 10 are already able to produce all the types of definition considered, the specific type of definition they produced critically depended on both the kind of the word to define and the specific knowledge domain it belonged to. In addition, there was a significant difference between groups of different ages.