Reciprocation is a principle that is fundamental to most internal combustion engines. Reciprocating means going back and forth over the same path, so a piston traveling back and forth in a cylinder is a reciprocating action.
A connecting rod is fastened at one end to the piston and at the other end to a crankshaft, which is in turn attached to a flywheel. This turns the vibrating back and forth reciprocating action into smooth and continuous rotary action.
The more reciprocating pistons and cylinders an engine has, the less vibration it will have and the more smoothly it will run.
