The term 'Chief Rival' can be defined as a person or group that is in competition with another person or group. In this case, the crossword solution for Chief Rival is RAIDER, which is a term used to describe someone who engages in plundering or pillaging. This term is often used to describe a person or group that is in competition with another person or group, such as a rival army or a rival business. Raiders are typically seen as a threat to the other person or group, and they often use tactics such as surprise attacks or raids to gain an advantage.