The 'base of support' is the 'platform' upon which an object stands or balances. The size, shape, density, and texture of this base plays an important part in the stability levels of objects supported by it:
Size: a narrow base tends to provide less stability than a wide one. Compare someone standing on a tightrope to one standing on a log.
Shape: take three differently shaped objects with the same amount of area on which to stand: an ordinary chair with four legs, a chair with an inclined seat, and a rocking chair with a rounded base. Each successive chair is more difficult to stand on due to the nature of its shape.
Density: this is the measure of the compactness of a material. In general, a denser base will offer more support than a sparse one. Compare standing on concrete with standing on a mattress.
Texture: this is the surface of a material as experienced primarily by the sense of touch; chiefly whether it's rough or smooth. Rougher surfaces grip better and thus offer more stable support. For instance, compare the stability one feels standing on grass with standing on smooth ice.
When skateboarding, there are at least three bases which must be taken into consideration:
a. the surface on which one's skateboard is on (i.e. a narrow metal rail, a curved wooden ramp, etc);
b. the skateboard itself which supports the skateboarder (i.e. is the grip tape worn down or still coarse);
c. the skater's feet/legs which serve as a base for his 'centre of gravity' located near the navel when one is standing naturally. (i.e. are the legs straight or bent, is the stance wide or narrow, etc).
To be continued...