We all know what a shape is. A square, a circle, a triangle…
In fact, any enclosed area created through lines, textures or colours is a shape. Here are some shapes:

Shapes can also be defined by other shapes, through variations in pattern or formation:

Or by negative shapes, where one shape is defined by missing pieces in other shapes:

A Kanizsa Triangle demonstrating the ambiguous nature of our perceptions. Image source Wikimedia Commons
All works of visual art contain shapes. Take this painting by Paul Gauguin, titled Arlésiennes (Mistral), in which shapes are the dominant element of the whole composition.