Kensington Security Slot
A Kensington Security Slot (also called a K-Slot or Kensington lock) is a small hole found on almost all recent small or portable computer and electronics equipment, particularly on expensive and relatively light ones: laptops and LCDs, and even some larger electronics, such as many recent Dell PCs. It is used for attaching a lock, in particular those from Kensington Technology Group, who are its originators.
Locks are generally secured in place with a key or some mechanical PIN device and attached with a strong metal cable to some permanent object, such as a heavy table or other similar equipment.
Kensington locks aren't designed to be a solid protection measure. As computer equipment bodies are generally made of plastic or thin metal, the lock can be torn out, heavily damaging the body. However, potential thieves trying to resell such stolen equipment would probably fail to do so: the broken Kensington lock hole would give them away. The Kensington lock is a good solution for busy offices, but given enough time and/or proper tools, thieves can easily circumvent it.
Several manufacturers offer similar mechanisms that don't require a special lock hole. They attach to a popular port, such as the VGA or printer port and have special screws to secure locks in place. These mechanisms are more universal, but occupy one port, so they're used mostly when Kensington lock holes are not available.