Cylinder is the word for a mechanical device that moves linear driven by air pressure.
The inside of the pneumatic cylinder is round so the sealing between the moving part, the piston, and the cylinder tube can be done sufficient. Attached to the piston is the piston-rod that sticks out of the cylinder through the " nose". I this nose are 2 seals; 1 for keeping the air inside and 1 for keeping the dirt outside. Between them there is a guide bushing. If there is a presssure at 1 side of the piston and no pressure at the other side, the piston will move and the piston-rod will move. That is way they are also called pneumatic actuators.
This is a simple and easy way to make a movement, only air pressure is needed. This is clean and not as dangerous as hydraulics.