Shape –  A two dimensional area created when a line is enclosed, or when there is a contrast with surroundings

Note: Leonardo Da Vinci represented a radical break from tradition when he realised that line didn't define form alone. Blending shadows and light in his painting still created shape but his paintings became more lifelike.

Shapes can be mathematical, like circles, squares and triangles:

Shapes can be free-form, following no rules:

Shapes can be recognisable as objects we see around us :

Shapes can be made to contrast with surroundings. M. C. Escher was an expert in it.