Wikimedia disambiguation page
The word straight means something that goes in one direction without turns, bends, or curves.
Lines can bend in a curve. If lines do not bend, they are straight. For example, the edge of a circle is not straight. The edge of a circle bends in a curve.
Light bends in a curve in air and water, but travels in a straight line away from the earth or other bodies in outer space where there is no air. The word "straight" is also a slang term for "heterosexual"