surface
/ˈsɜːfɪs/
The overside or up-side of a flat object such as a table, or of a liquid.
The outside hull of a tangible object.
Outward or external appearance.
On the surface, the spy looked like a typical businessman.
The locus of an equation (especially one with exactly two degrees of freedom) in a more-than-two-dimensional space.
(fortification) That part of the side which is terminated by the flank prolonged, and the angle of the nearest bastion.