A shape on a flat picture surface that is defined by surrounding empty space is known as ________ shape.

A shape on a flat picture surface that is defined by surrounding empty space is known as a negative shape. Negative shapes, also referred to as negative space, are crucial in art as they help to define and organize the composition of positive shapes (the actual objects or figures being represented).

In visual arts, negative space is the area that surrounds the positive shapes, and it can often create interesting designs and visual relationships between objects. Recognizing negative shapes can enhance one's ability to create balance and harmony in artwork. Artists often pay close attention to this concept not just for the sake of aesthetics, but also for effective storytelling and to draw the viewer’s attention to the central subjects.

In summary, the answer is negative shape.