Which of the following is NOT a type of altitude?

Relative altitude is not typically recognized as a standard type of altitude in aviation. The three primary types of altitude used in aviation are indicated altitude, pressure altitude, and true altitude.

Indicated altitude refers to the altitude read directly from the altimeter when it is set to the local altimeter setting. Pressure altitude is the altitude at which the pressure is equal to the standard atmospheric pressure at sea level (29.92 inHg). True altitude, on the other hand, is the actual altitude above mean sea level, which may vary depending on atmospheric conditions.

While the term "relative altitude" can be used in certain contexts, particularly in military or specific operational settings to describe altitude in relation to a certain reference point, it does not fall under the standardized definitions of altitude utilized in aviation regulations and practices. Therefore, it is not considered a formal type of altitude like the others listed.