A shape is the form of an object or its external boundary, outline, or external surface, as opposed to other properties such as color, texture, or material composition.
Psychologists have theorized that humans mentally break down images into simple geometric shapes called geons.〔Marr, D., & Nishihara, H. (1978). Representation and recognition of the spatial organization of three-dimensional shapes. Proceedings of the Royal Society of London , 200, 269-294.〕 Examples of geons include cones and spheres.
== Classification of simple shapes ==
(詳細はpolygons are classified according to their number of edges as triangles, quadrilaterals, pentagons, etc. Each of these is divided into smaller categories; triangles can be equilateral, isosceles, obtuse, acute, scalene, etc. while quadrilaterals can be rectangles, rhombi, trapezoids, squares, etc.
Other common shapes are points, lines, planes, and conic sections such as ellipses, circles, and parabolas.
Among the most common 3-dimensional shapes are polyhedra, which are shapes with flat faces; ellipsoids, which are egg-shaped or sphere-shaped objects; cylinders; and cones.
If an object falls into one of these categories exactly or even approximately, we can use it to describe the shape of the object. Thus, we say that the shape of a manhole cover is a disk, because it is approximately the same geometric object as an actual geometric disk.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』