A gall is a growth on the surface of plants and other lifeforms. They are caused by various parasites, such as fungi, bacteria, insects and mites.
Plant galls are abnormal growths of plant tissues similar to benign tumors or warts in animals.
What causes a plant gall can often be known without the actual agent being there. Many types of gall have typical structures. This is true of many galls produced by insects and mites.