Radius is the distance from the center of a circle to the outside edge.
The relationship between the radius and the circumference of a circle is .
Starships often scanned within a given radius when making an analysis or evaluation of nearby space.
In the alternate timeline created by the Na'kuhl, Nazis searched for Captain Archer within a fifteen-mile radius. (ENT: "Storm Front")
In 2346, while attempting to assassinate Gul Dukat, the Bajoran resistance gave Major Kira Nerys (as Luma Rahl) a Bajoran earring made of ultritium resin which, when detonated, had a blast radius of twenty meters. (DS9: "Wrongs Darker Than Death or Night")
After failing to obtain a transporter lock on the away team on Trelane's Gothos, Spock beamed aboard all lifeforms within a predetermined radius. (TOS: "The Squire of Gothos")
The 22nd century Vulcan hand scanner was capable of scanning within a three kilometer radius. (ENT: "Carpenter Street")
In the mirror universe, Doctor Phlox measured the bite radius of a bite mark left behind by an unknown reptilian and determined it was likely at least two meters in length. (ENT: "In a Mirror, Darkly, Part II")