Medieval grammar recruited the Latin concept word affectus to account for aspects of language structure and language use not explicitly or easily dealt with otherwise by grammatica’s traditional theory and terminology for articulation, morphology, parts of speech, and syntax. Medieval grammar inherited the descriptive terminology of Latin grammatica, but grammarians also invented a number of new theoretical and descriptive terms and concepts, including affectus, by adopting philosophical, especially Aristotelian, terms and concepts as well as by appropriating existing Latin words (such as regimen, dependere). In early medieval grammatical discourse, affectus is principally found in descriptions of the interjection (interiectio) as a part of speech, for example, emotive words such as ‘Ouch!,’ ‘Alas!,’ or ‘OMG!.’ Less frequently, the phrase affectus animi (or mentis) occurs in translations of Aristotle’s Peri Hermeneias received through Boethius’s Latin commentary. After the ninth century CE, the renewed influence of Priscian’s Institutiones grammaticae in grammatical discourse gradually enhanced affectus as a theoretical concept in the philosophy of mind, language, and reality, especially discussions of the communicative status of well formed, incomplete, or figurative constructions. In the thirteenth century, some commentators and philosophers of language distinguished signification under the modus affectus from signification under the modus conceptus to account for speech and behaviour stating a case (actus significatus) or functioning as a performative (actus exercitus).