The prefix 'INTERNAL' is used to describe something that is located or occurring WITHIN a certain area or entity. The two crossword solutions for this term are 'endo' and 'ENTO'. Endo is a prefix derived from the Greek word 'endos', meaning 'WITHIN'. It is commonly used in medical terminology to refer to something that is inside the body, such as an endoscope. ENTO is also a Greek-derived prefix, meaning 'WITHIN'. It is often used to refer to something that is inside a larger structure, such as an entomology study of insects WITHIN a particular ecosystem.