||The Catalan philosopher and theologian Ramon Llull (Raimundus Lullus, ca. 1232-1316) is well known for his Ars: a philosophico-theological system which takes the basic concepts of the three monotheistic religions of its time (such as God’s attributes, the virtues and vices, etc. ) and combines them in mechanical figures (i. e. rotating circles) in order to convince Muslims (and Jews) of the (logical) superiority of Christianity and, so, to convert them to the Christian faith. One of the peculiarities of Llull’s Ars is its very innovative use of logical devices, the most famous being the quasi-algebraic notation of its principles (each principle being represented by a letter, so that they can easily be combined). Among these innovative achievements stands Llull’s curious theory of definition, which will be presented and discussed in this paper. Our first consideration shall be to give a brief account of the Aristotelian setting and the Arabic reception of Aristotle’s concept of definition (I. ); from here we shall proceed to show how Llull’s theory of definition originates from within the context of the Aristotelian Ορος διαιρετικóς and its Arabic interpretation (II. ); finally we shall draw some connecting lines between Llull’s definitions, modern theories of definition themselves and Russell’s and Quine’s theory of descriptions (III. ).