Concentric objects share the same center, axis or origin with one inside the other. Circles, tubes, cylindrical shafts, disks, and spheres may be concentric to one another. Concentric objects generally have different radii, as concentric objects with the same radius are equal.
One of the most familiar examples of concentric circles are the evenly spaced circles of a target used in target archery or firearms, and accordingly any concentric design may be called a "target" or a "bullseye" (after the center).
Concentric cable is a cable type used in TN-C-S earthing systems where the combined neutral and earth core completely surrounds the live core(s). This minimizes the risk of completely cutting through the combined neutral and earth without first completely cutting through the live cores.
Concentric muscles contractions are those in which the muscle shortens.