The study wants to contribute to the explication of Isidore of Sevilla’s conception of etymology starting with the definition “Etymologia est origo vocabulorum, cum vis verbi vel nominis per interpretationem colligitur” (Etym. I, xxix, 1). At first sight the problem of interpretation results from the contradiction between the static character of the principal clause (est origo)and the dynamic character of the subordinate clause (colligitur). This contradiction is resolved in favour of the dynamic aspect by changing origo into originatio on the basis of a teleological and historical interpretation. In a second step, an attempt has been made to arrive at a comprehensive assessment of the definition by interpreting the constituents of the subordinate clause and by defining its relation with the principal clause. The first question concerns the function of the conjunction cum (the hypothesis of a conditional nexus is rejected from the first); in the course of the examination of this problem the constituents of the subordinate clause are analyzed (in particular interpretatio and vis). As to interpretatio it is shown that it corresponds to the justification of the origo (“Stellae dictae a stando, quia fixae stant in caelo nec cadunt”); vis is then described as a relational notion that is defined by the relation between the semantic contents of the origo and an actual characteristic property of the thing. Finally an examination of Isidore’s actual proceeding demonstrates that the knowledge of vis results immediately from the knowledge of the origo (originatio) and the interpretatio. Consequently, the conjunction cum can be defined as cum coincidens.