The path followed by a point or line when moving.

For example, a line moving in a circle traces out a cylinder.

In the applet on the right, press "Run". The line on the right is called the generatrix. As it moves at a fixed distance from the left line, it traces out a right circular cylinder. The path it follows is a circle. The path followed in the way is called the directrix.

Pages referring to 'directrix'

A cylinder is a closed solid that has two parallel (usually circular) bases connected by a curved surface. It can be a right cylinder or an oblique cylinder.
Definition a parabola as the locus of all points equidistant from a given point and a line.
A cylinder can be thought of as a right regular prism with an infinite number of sides.