Vesicle: In dermatology, a vesicle is a small blister, as on the skin. Vesicles also occur on the mucous membranes, such as the buccal mucosa (the lining of the mouth). Vesicles are less than 0.5 centimeters in diameter. In anatomy, a vesicle is any small pouch. For example, the seminal vesicles are a pair of pouch-like structures about five cm (2 inches) long located behind the bladder and above the prostate gland that contribute fluid to the ejaculate.