Other forms: radii
The radius is the line from the middle of a circle out to its edge — half the way across. Cut a pizza from the centre and every cut you make is a radius.
Double the radius and you have the diameter. The plural is radii, said RAY-dee-eye, and that is the form you will meet in print. Away from mathematics, a radius is a round area measured outwards from a point: every shop within a five-mile radius, all the damage inside a radius of two hundred metres. In your arm the radius is the shorter of the two forearm bones, the one on the thumb side. Radiate and radiation belong to the same family, all of them about something spreading out from a centre.
1nounradiusThe straight line from the centre of a circle or sphere to its edge; half the diameter.
“If the radius is four centimetres, the circle is eight across.”
2nouncircularA circular area measured outwards from a central point.
“Every house within a radius of two kilometres lost power.”
3nounboneThe bone on the thumb side of the forearm.
“She fell off the ladder and fractured her radius.”
radius is a measurement of a circle.
Radius is Latin for 'ray', taken into English unchanged, spelling and all. The same Latin word came into English by a second and older road, worn down along the way into ray. A radius and a ray of sunlight are the same word twice.
Latin
radius
English, today
radius
Borrowed from Latin radius. Latin radius (“ray”). a doublet of ray.
Etymology from Wiktionary, CC BY-SA