I believe linear motion is simply straight motion because linear means straight

Linear motion is motion that occurs along a straight line, and therefor non linear motion is motion that does not.

Refer to the Wikipedia article in the Related links section below.

1. Cam and follower mechanism: Converts rotary motion into linear motion by translating the motion of a cam into the linear motion of a follower.
2. Rack and pinion system: Uses a rotating gear (pinion) to move a linear rack back and forth, converting the rotary motion into linear motion.
3. Scotch yoke mechanism: Utilizes a circular motion to drive a sliding block in a straight line, converting rotary motion to linear motion.
4. Lead screw mechanism: A rotating screw that moves a nut along its threads, translating rotational motion into linear motion.

A linear actuator is a device that creates linear motion, often by converting rotational motion. For instance, an electric linear motor turns a screw that in turn "pushes" a cylinder. By this action, linear motion is created from a rotating shaft.

linear motion.