The present paper aims to investigate the main argumentative uses of definitions in various communicative contexts of the parliamentary discourse, on the basis of two sets of data, selected from the British and the Romanian Parliaments. Relevant categories of argumentative definitions are identified and described, by taking into consideration their linguistic structure and rhetorical features, as well as their current association with other types of arguments and pragmatic strategies. The cross-cultural and comparative perspective allows to grasp to what extent the institutional forms, procedural rules and cultural models can actually influence the argumentative choices and reasoning patterns in the specific cases of the British and the Romanian Parliamentary discourse.