In geometry, a right line drawn or extending from the center of a circle to the periphery, and hence the semidiameter of the circle. In trigonometry, the radius is the whole sine, or sine of degrees.

In anatomy, the exterior bone of the fore arm, descending along with the ulna from the elbow to the wrist.

In botany, a ray; the outer part or circumference of a compound radiate flower, or radiated discous flower.