What term indicates that you have an aircraft that is not within the formation in sight?

The term used to indicate that an aircraft is not within the formation in sight is typically "blind." This indicates that a pilot is unaware of the position and status of their flight lead or wingman, meaning they cannot see them visually. In the context of formation flying, clear communication is essential for maintaining safety and effective coordination, and using the correct terminology helps ensure that all pilots are on the same page.

"Tally ho" is used to indicate that a pilot has a visual on the target or the nearest flight member, which is the opposite of what is being asked in the question. Similarly, "No Joy" indicates that the pilot is unable to locate the target despite looking for it, but it is not specifically tied to the formation context as "blind" is. Therefore, "blind" is the most accurate term to describe not having an aircraft in sight within the formation.