The thickened part of an underground stem of a plant containing buds from which new plant shoots (stems and young leaves of a new plant) grow. Potatoes are the most well known example of a tuber. Note the distinction between a root crop(things like beets or carrots, in which it is the actual root which is harvested) and a tuber crop (things like potatoes or sweet potatoes, in which the outgrowth is not a root, but a portion of the plant which provides the nourishment to new growth).