I
In geometry, a two-dimensional collection of points (flat surface), a three-dimensional collection of points whose cross section is a curve (curved surface), or the boundary of any three-dimensional solid.
In general, a surface is a continuous boundary dividing a three-dimensional space into two regions. For example, the surface of a sphere separates the interior from the exterior; a horizontal plane separates the half-plane above it from the half-plane below. Surfaces are often called by the names of the regions they enclose, but a surface is essentially two-dimensional and has an area, while the region it encloses is three-dimensional and has a volume. The attributes of surfaces, and in particular the idea of curvature , are investigated in differential geometry .
II
Outermost layer of a material or substance.
Because the particles ( atom s or molecule s) on the surface have nearest neighbours beside and below but not above, the physical and chemical properties of a surface differ from those of the bulk material; surface chemistry is thus a branch of physical chemistry . The growth of crystal s, the actions of catalyst s and detergent s, and the phenomena of adsorption , surface tension , and capillarity are aspects of behaviour at surfaces. The appearance of the surface, whether achieved with electroplating , paint , oxidation-reduction , bleaching (see bleach ), or another means, is aesthetically important.