a line that is perpendicular to another line or plane; also called [[垂线|垂線|chui2 xian4]]

the fixed straight line used to define a conic section, such as a parabola

(mathematics) two lines that are not parallel and do not intersect because they lie in different planes

An acceleration device made of many small tubes connected in a straight line that uses alternating voltage to accelerate particles like electrons and protons.

a line pointing straight down toward the center of gravity; any line that is perpendicular to a horizontal plane