(N|n)KRO, n-key rollover: key rollover is the minimum of the maximum number of simultaneous key presses that can be pressed at any one time and all register on the host computer.
NKRO over USB: the USB HID protocol by default only allows for 6 keypresses to be transmitted at any given time. There are various workarounds for this.
Keycap stem: The center part of the keycap that allows attachment to a switch stem.
Slider: Synonymous with stem. "Slider" is often seen in reference to Alps switches. Sliders/stems are the component of the keyswitch that moves up and down and is directly responsible for actuation.
Programmability: The ability to assign specific functions to a key on a keyboard. Allows for custom layouts. Seen on many Access-is matrix keyboards, and in limited functionality on many gaming keyboards.
Tactile: A switch that has a distinct snap or bump at the actuation point. For a rough example, a clicky pen is technically a tactile switch. Common examples include Cherry MX Blues, Clears, Whites, Greens, and Browns as well as White, Blue, Cream, and Orange Alps.
Linear: A switch that does not demonstrate any tactility- you get a constant, smooth stroke from it. Examples would be Cherry MX Blacks or Reds, as well as Green Alps.
Clicky: A switch that produces an audible click upon depressing it. Examples include Cherry MX Blues, Whites, Greens and White, Blue, and Orange Alps.
Non-Clicky: Switches that do not produce any additional sound aside from bottoming out and friction. Examples include Cherry MX Browns, Clears, Blacks, and Reds, as well as Cream Alps.
Silent/Silenced: Often seen in reference to specific Topre boards, these are switches that are specifically designed to be quieter than the normal version of the switch.