Short for Zero Insertion Force socket, the ZIF socket was designed by Intel and included a small lever to insert and remove the computer processor. Using the lever allows you to add and remove a computer processor without any tools and requires no force (zero force). All processor sockets from the Socket 2 and higher have been a ZIF socket design. The picture shows an Intel OverDrive socket and an example of a ZIF socket.
Note: During the evolution of the processor Intel did use a slot processor.