What is the difference between a vesicle and a bulla?

If the blister is less than 5 mm in diameter, it is referred to as a vesicle; if it is 5 mm or larger, it is called a bulla. Some dermatologists require that blisters be 1 cm before using the term bulla. The term vesiculobullous disorder is often used clinically since some patients may have both vesicles and bullae.