A single acting cylinder is an actuator for a fluidic system like a Pneumatic or Hydraulic system.

It consists of a plunger which allows a pressurized fluid to get behind it. This will push out the piston, and has a spring on the other side of the plunger which will return it if the pressure is taken off.

Note that not all have any return mechanism, and this may have been down to having an opposing cylinder to go the other way, or merely to be pulled down by gravity. Some also used vacuum suction to return.

These are somewhat less controllable than a Double Acting Cylinder. They are perhaps less often used than they were. There was a time when these were the only type of cylinder available from Lego in their pneumatic range.