The term 'Cocoon Stage' refers to the fourth stage of the life cycle of a butterfly, which is known as the PUPA stage. During this stage, the caterpillar forms a protective shell or cocoon around itself and undergoes a process of metamorphosis, transforming into an adult butterfly. The PUPA stage is the longest stage of the butterfly's life cycle, and it is during this time that the caterpillar develops its wings and other adult features.