A return spring is a spring in a keyswitch that keeps the switch open until it is pressed and returns the slider to its home position after the switch is released. Typically this is a helical (standard spiral) spring.
The return spring is responsible for at least part of the force exerted against the operator's finger. Tactile and clicky switches frequently use separate leaf springs that provide a significant amount of extra force. The contact mechanism is also frequently a leaf spring, and depending on the design (especially with Cherry MX switches) can also exert force against the slider. The return spring alone is typically linear.
The combination of slider and return spring has been largely replaced with rubber domes.