Water blister: A blister with clear watery contents that is not purulent (does not contain pus) and is not sanguineous (does not contain blood).
A blister is medically termed a vesicle. One that is more than 5 mm in diameter with thin walls and is full of watery fluid is called a bulla.
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.