The ability to manage specific forms of disciplinary expression - Languages of Schooling - is regarded as a factor of academic success (Council of Europe recommendations - Council of Europe CM/Rec. [2014]. "Recommendation of the Committee of Ministers to Member States on the Importance of Competences in the Language(s) of Schooling for Equity and Quality in Education and for Educational Success." Accessed December 25, 2018. https://search.coe.int/cm/Pages/result_details.aspx?ObjectID=09000016805c6105). One of the core discursive functions students perform across academic subjects is defining, which is part of the inventory of descriptors for the language of schooling (e.g. Beacco [2010]. "Items for a Description of Linguistic Competence in the Language of Schooling Necessary for Teaching/Learning History (End of Obligatory Education)." Strasbourg: Language Policy Division, Council of Europe). This study addresses defining as a component of the language of schooling by which CLIL students express specialized knowledge across languages, educational levels and fields (see Coffin [2006b]. "Historical Discourse. The Language of Time, Cause and Evaluation." London: Bloomsbury Publishing, Continuum). We elicited, analysed and compared students' written definitions in English (L2) and Spanish (L1) of two different historical fields in primary (6th grade) and secondary (8th grade). For this purpose, we applied an analysis scheme that merges Trimble's ([1985]. "English for Science and Technology: A Discourse Approach." Cambridge: Cambridge University Press) definitional construct and Systemic Functional Linguistics (Halliday and Matthiessen [2014]. "An Introduction to Functional Grammar." London: Hodder; Martin [2013]. "Embedded Literacy: Knowledge as Meaning." "Linguistics and Education" 24: 23-37). Our results show that while students produced more definitions in English in the higher educational level, the differences in their realizations are attributed more to the field being defined. The study has also shown no differences in the frequency and type of definitions across languages.