The term 'Skeleton Prefix' refers to two prefixes, EXO and ENDO, which are used to describe the relationship between a structure and its surrounding environment. EXO is a three-letter prefix meaning 'OUTSIDE, ' while ENDO is a four-letter prefix meaning 'INSIDE.' These prefixes are commonly used in biology to describe the relationship between an organism and its environment, such as the relationship between a bone and its surrounding tissue. They can also be used to describe the relationship between a structure and its environment, such as the relationship between a building and its surroundings.