The term 'chain segment' can be defined as a single LINK in a chain. It is the smallest unit of a chain, and is usually composed of metal or plastic. A chain segment is often used to connect two objects together, such as a bicycle chain connecting the pedals to the wheels. The crossword solution for this term is 'LINK', which is a four-letter word. A LINK is a connection between two things, and in this case, it is referring to the connection between two chain segments.