Also called point of demarcation, demarc extension, or demarc, it is the physical point at which the public network of a telecommunications company (i.e., a phone or cable company) ends and the private network of a customer begins - this is usually where the cable physically enters a building.
For example, if your business subscribes to receive Internet through a local phone or cable company, the provider installs a demarcation point at your business. The demarc is usually a grey box mounted on the outside of the building. On the outside of the building the provider's wires run in to the demarc (overhead or underground). The wires are switched at the Demarc and are then run into the physical building. For residential services, the demarc is often installed inside the residence in the homeowner's cellar or basement.