at some point through the clutches travel it is "slipping"
when you leave from a stop in first gear, you will slip the clutch untill you are moving at a speed where the engine can turn itself over using the intertia of the car.
so, that magic spot on the travel of the clutch that you hold your foot at while leaving the line, is the slip point, or friction point of the clutch, its the spot where the clutch is doing what its designed to do.
when the clutch is doing what its designed to do, its slipping. usually.
when most of the people here are talking about slipping the clutch, its usualy related to how or when they slip the clutch to achive a fast "launch". a launch is an attempt to get the car to leave from a stop as quickly as posible. as aposed to dumping the clutch.