What is defined as a straight line?

A straight line is commonly defined in geometry as "linear." This term specifically refers to a line that is perfectly straight, extending infinitely in both directions without any curvature. Linear attributes are fundamental in various contexts of mathematics and physics, establishing a baseline for definitions and calculations involving distance, angles, and other geometric principles.

While “curvilinear” describes lines that are curved rather than straight, and "rectilinear" pertains to motion or figures that consist of straight lines, "geometric" is a broad term encompassing all shapes and figures, including both straight lines and curves. In this context, "linear" directly captures the essence of a straight line, making it the most accurate choice for this definition.