Something that's basal forms the base or the bottom layer of an object. The basal leaves on a plant are connected to the lowest section of its stem.
In medicine, the word basal describes structures that are located at the base of some major organ or other body part. Basal ganglia, for example, are found at the base of the brain in humans, and a basal cell is at the deepest part of the skin. Basal comes from the word base, from the Latin basis, "foundation," via the Greek basis, "step or pedestal."