In a manual transmission, the shaft running parallel to the main shaft and carrying the gear pairs is the countershaft.
The countershaft, also called a layshaft, is an intermediate shaft in a typical constant-mesh manual gearbox. Its gears are permanently engaged with corresponding gears on the input shaft and with gears associated with the output shaft. Selecting a gear locks the appropriate output-side gear to its shaft, creating the chosen ratio.
This arrangement lets a manual transmission offer several ratios while keeping the gears meshed. Synchronizers and sliding collars help match speeds and engage the selected gear smoothly. In many longitudinal transmissions, the input and output shafts share an axis, while the countershaft sits parallel to them.
“Input shaft” and “output shaft” describe different parts of the power path, so they are not interchangeable answers. The input shaft receives torque from the clutch and engine; the output shaft sends torque toward the final drive. Some front-wheel-drive transaxles use a countershaft that also performs the output-drive function, but the intermediate gear-carrying shaft is still conventionally called the countershaft.