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, lets you add and remove a computer processor without any tools and requires no force (zero force). All processor sockets since Socket 2 utilize a ZIF socket design. The picture shows an Intel OverDrive socket and an example of a ZIF socket.
During the evolution of the processor, Intel did use a slot processor.