What defines a cookoff in mortar operations?

A cookoff in mortar operations is defined as a situation where the mortar round prematurely detonates due to excessive heat in the barrel. This phenomenon occurs when the temperature inside the barrel rises to a level that ignites the propellant in the round, resulting in an unintended explosion. This can be particularly dangerous as it may occur without the operator's command and can lead to severe consequences, including injury to personnel and damage to equipment.

The other options do not accurately capture the essence of a cookoff. A delay before firing refers to a planned aspect of the firing sequence rather than an unintended detonation caused by heat. A secondary explosion indicates a follow-up blast that may occur due to various factors but is not synonymous with a cookoff. Lastly, when a round does not leave the barrel relates to a failure to fire situation rather than the spontaneous detonation characteristic of a cookoff. Understanding these distinctions is crucial for safe mortar operations and effective equipment management in the field.