Differing definitions of multilingualism arise on account of two related groups of reasons: those deriving from participants’ complex situation with regard to the nature of their use of various languages, and those deriving from researchers’ differing backgrounds, ideologies and purposes. Most psycholinguistic researchers define multilingualism as the use of three or more languages, but this entails defining what a language is, which can be problematic.
 Researchers need to decide on the degree of proficiency and functional capability multilinguals are required to have for a language to count in their study, and weigh up the implications of psycholinguistic (eg, mutual intelligibility and psychotypological perception), cultural, political, and affective criteria, together with literacy. Researchers should specify what they mean by ‘multilingual’.