From Teflpedia

A prefix (/prefɪks/) is an affix, specifically an adfix, added to the beginning of a stem, or word, to modify its meaning (e.g. ex-, dis-, un-).

While easier for speakers of European languages, and harder for students from other regions of the world, language learners should be encouraged to use word analysis to work out the meaning of unfamiliar words and teachers can make use of several types of exercises to practise this skill.