In geometry, a "선 (Line)" is a straight one-dimensional figure that extends infinitely in both directions. It has no thickness and is defined by two points, which are called endpoints. Lines are fundamental in mathematics and are used to create shapes, angles, and various geometric figures. In art and design, a "선 (Line)" can refer to a mark made by a tool, such as a pencil or brush, that connects two points. Lines can vary in width, length, and style, and they play a crucial role in creating visual compositions, guiding the viewer's eye, and conveying emotions.