What does "contour" mean?

Contour: (Mil.) The outline of a horizontal section of the ground, or of works of fortification. Contour feathers (Zoöl.), those feathers that form the general covering of a bird. -- Contour of ground (Surv.), the outline of the surface of ground with respect to its undulation, etc. -- Contour line (Topographical Suv.), the line in which a horizontal plane intersects a portion of ground, or the corresponding line in a map or chart.