A region of the two-dimensional Cartesian plane that is bounded by a line is known as a half-plane. The line, termed the boundary, divides the plane into two such regions. If the boundary line is included in the region, it is a closed half-plane; otherwise, it is an open half-plane. For example, the set of all points (x, y) such that y > 0 represents an open half-plane, while the set of all points (x, y) such that y 0 represents a closed half-plane.
This concept is fundamental in diverse areas of mathematics, including linear programming, optimization, and geometric analysis. Its importance stems from its ability to describe regions of feasibility and constraint satisfaction. Historically, the formalization of this idea has roots in the development of analytic geometry and the study of inequalities. Its use simplifies the representation and analysis of many mathematical problems, providing a clear and concise way to define and manipulate planar regions.