The word "co-pilot," also known as the second-in-command of an aircraft, can be substituted with various synonyms. For instance, "first officer," "assistant pilot," "flight officer," and "deputy pilot" are some common terms that can be used instead of co-pilot. Other synonyms for co-pilot include "second pilot," "backup pilot," and "co-flight crewmember." Additionally, some airlines use specific terms like "relief pilot," "pilot flying," or "pilot not flying" to designate the co-pilot's duties during a flight. Regardless of the term used, the co-pilot's primary role is to assist the captain and maintain a safe flight operation.