A Prusik is a friction knot used to grip a section of rope. It is typically used for ascending ropes or providing a backup lock to a belay device.
Named after Austrian mountaineer Karl Prusik. A prusik typically involves looping a thinner diameter cord around a thicker cored or rope. In this way the prusik can grip the rope or cord. Prusiks only "grip" the rope when loaded, so they can be slid up or down a rope. They are generally used for ascending vertical rope or as a back up to a belay device (i.e. if the user of the belay looses their grip the backup will prevent the rope from feeding unhindered)