This paper presents an automatic Generator of dictionary definitions for concrete entities, based on information extracted from a Computational Lexicon (CL) containing semantic information. The aim of the adopted approach, combining NLG techniques with the exploitation of the formalised and systematic lexical information stored in CL, is to produce well formed dictionary definitions free from the shortcomings of traditional dictionaries. The architecture of the system is presented, focusing on the adaptation of the NLG techniques to the specific application requirements, and on the interface between the CL and the Generator. Emphasis is given on the appropriateness of the CL for the application purposes.