A leased line is a dedicated communication channel that interconnects two or more sites. This is a service contract between a customer and a provider. It acts as a dedicated tunnel from one point to the other where data can continuously flow for a fixed monthly fee or rent, hence the name. Leased lines are used for Internet, data and even telephone services. They are typically run on fiber optic cables to provide large bandwidth and speed.
A leased line is not really a dedicated physical connection, but a reserved circuit between two designated points that is open at all times. This is unlike traditional telephone services, which reuse the same circuit through switching. They are typically rented by large companies to connect two or more sites that need constant fast connection. These lines are leased by large telecommunication companies and are generally quite expensive. The alternative to this is to use the public switched networks while using security protocols, or to install and maintain their very own private lines, which can be more expensive.