Compression Stroke: One of the four 180-degree full "sweeps" of the piston moving in the cylinder of a four-stroke, internal-combustion engine (originally devised by Nikolaus Otto in 1876). During the compression stroke, the piston moves from BDC to TDC and compresses the air/fuel mixture. Note: The 180-degree duration of the compression stroke is commonly longer than the duration between the intake valve-closing point and top dead center or ignition, sometimes referred to as the true "Compression Cycle." The compression stroke is followed by the power stroke.