Definition - What does Gall mean?
Galls are essentially plant tumors or abnormal growths on any part of a plant. While most often galls are the result of insects, other parasites such as fungi, bacteria, mites, or nematodes may be the initiator.
The scientific study of plant galls is called cecidology.
MaximumYield explains Gall
Galls do not inhibit normal plant growth or kill the plant, but are rather like benign tumors or warts; growths on the epidermal layer or bark of the plant.
The structure of plant galls is such that they are highly organized growths where the actual cause of the growth can be identified without the exact contributor being known.
Apple trees are particularly susceptible to the growth of galls.