a line that passes through the center of curvature of a lens so that light is neither reflected nor refracted | the center around which something rotates |
in a doubly refracting crystal, the line in the direction of which no double refraction occurs | the 2nd cervical vertebra; serves as a pivot for turning the head |
| a straight line through a body or figure that satisfies certain conditions |
| a group of countries in special alliance |
| the main stem or central part about which plant organs or plant parts such as branches are arranged |