Travel, actuation and hysteresis
The vertical motion of a pushbutton switch is called its “travel” (German “Hub” or “Weg”). “Actuation” means registering that the switch has been pressed; depending on the type of switch, this may mean that it starts to conduct electricity (which is the most common behaviour) but there are other ways for the keyboard to detect keypresses.
The term “full travel” is not clearly defined. RAFI use “full travel” for switches with 2.5 mm of travel or more, yet they consider 4 mm travel to be suitable for data entry keyboards. Laptop keyboards have travel as low as 1 mm (with the Sony VAIO VPCZ21M9E 13″ notebook).
Pretravel is the distance that you can press down on a key before it actuates.
Overtravel is the additional distance that the key can continue to be pressed after actuation. Overtravel means that keys do not have to be struck hard or pressed all the way down. From the patent for General Instrument S700 Series:
In a keyswitch with overtravel, electrical contact is made (i.e., the switch turns “on”) before the keytop is fully depressed. Overtravel provides for more reliable switch operation, because keyboard operators often inadvertently fail to press the keyswitch down the whole way, particularly when typing at a high rate of speed. When overtravel is provided, the keyswitch will turn on as long as the plunger is depressed at least to the point where electrical contact is made.
In many switches, if you press the switch slowly up to the point that it actuates, and then begin to let go, it instantly stops registering. This is (or at least was) considered by many to be weakness, that would allow as switch to be “teased”: if the operator hesitates just at that point, multiple keystrokes could be inadvertently generated. From the S700 patent again:
If the make and break points in the switch are at the exact same physical location on the downward and upward strokes (i.e., no hysteresis), it will be possible for a keyswitch operator to unintentionally produce multiple actuations if any hesitation is made at the exact moment the switch turns on. This phenomenon is known as “teasing” …
Granted, this period of hesitation would have to be very short, as most systems now implement electronic autorepeat.