What are often used to define the edge or boundary of something?

Unity

Lines

Texture

Rhythm

The answer is "Lines."

Explanation:

A "line" is considered one of the fundamental elements when it comes to "art." It comes in different forms such as "straight lines," "curved lines," "diagonal lines," "spiral lines," etc.

When it comes to certain situations such as "creating shapes," the lines are used to define the edge or boundary of something. As a boundary, the line functions to circumscribe the shape (such as a circle).

Thus, this explains the answer.