During the last few years, a lot of researches have focused on automatic definition extraction in the context of question answering systems. Although, these researches have been conducted for different languages, no research has been proposed for Arabic. In this paper, we tackle the automatic definition extraction in the context of Question Answering systems. We propose a method based on patterns to automatically identify a definition answer to a definition question. The proposed method is implemented in an Arabic definitional question answering system. We experimented this system using a set of 50 definition questions, and a corpus of 2000 snippets collected from the Web. The obtained results are very encouraging: 94% of the definition questions have complete definitions among their first 5 answers.