The term PREFIX FOR 'VENOUS' is INTRA, which is a five-letter prefix. Intra is derived from the Latin word 'intus, ' meaning 'WITHIN, ' and is used to describe something that is located or occurring WITHIN a certain area or space. In medical terminology, it is used to refer to something that is located inside the body, such as a vein or artery. Therefore, INTRA is the prefix for 'VENOUS', which refers to veins.