Blister: A collection of fluid underneath the top layer of skin (epidermis). One that is more than 5 mm in diameter with thin walls and is full of watery fluid is called a bulla or a bleb. There are many causes of blisters including burns, vesicant agents, friction forces, and diseases of the skin.

There are a number of types of blisters, including:

• Blood blister -- a blister full of blood due to a pinch, bruise or repeated friction.
• Water blister -- a blister with clear watery contents that is not purulent (does not contain pus) and is not sanguineous (does not contain blood).
• Fever blister -- a blister in the mouth or around it that causes pain, burning, or itching before bursting and crusting over. It is due to the herpes simplex virus which is latent (dormant in the body) and can be reawakened (reactivated) by such factors as stress, sunburn, or fever. Hence, it called a fever blister or a cold sore.

The word "blister" entered English in the 14th century. It came from the Middle Dutch "bluyster", blister and was a modification of the Old French "blostre" which meant a leprous nodule -- a rise in the skin due to leprosy.