Emily Dickinson Lexicon

RA'DI-US, n. [L. id, a ray, a rod, a beam, a spoke, that is, a shoot; radio, to shine, that is, to dart beams. See Ray.]

1. 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 90°.
2. In anatomy, the exterior bone of the fore arm, descending along with the ulna from the elbow to the wrist.
3. In botany, a ray; the outer part or circumference of a compound radiate flower, or radiated discous flower. – Martyn.