The prefixes ENTO and endo both mean 'INSIDE' and are used to form words in many languages. ENTO is derived from the Greek word entos, meaning 'WITHIN, ' while endo is derived from the Greek word endon, meaning 'WITHIN.' Both prefixes are used to describe something that is INSIDE or contained WITHIN something else. For example, an entomologist is someone who studies insects, and an endocrinologist is someone who studies hormones.