The grooves on a shaft that line up with and engage another shaft or gear so that one rotates the other.

Ridges machined or cast onto the outer diameter of a shaft or inner diameter of a bore to enable parts to mate without rotation.

The internal or external longitudinal grooves in a gear or shaft that mesh when assembled, causing the gear to turn with the shaft, but allow lateral movement.

are mathematical representations of curves. Splines are used to create objects such as irregular curves, which can't realistically be represented by straight lines.