A contour refers to the outer boundary of a shape. I think of it as a two-dimensional concept. The contour of a sphere is the circle its boundary creates. The quality of a contour can vary from sharp/defined to soft/atmospheric, depending on the subject matter. The contour of hair is often soft while the contour of a face is usually sharp and defined.

Contour drawing is a foundational exercise in artistic training where you practice following the outer boundary of a subject. Blind contour drawing is a variation of this exercise where you draw the contour of a subject without looking at the paper.