A telephone line leased expressly for the purpose of connecting two users more-or-less permenantly.. Such lines may be "voice grade" which provides the bandwidth and signal to noise ratio of ordinary public switched telephone network circuits, or specified in ways which allow transport of suitably encoded digital signals at faster rates.
In some cases, lines may be physical wires between the communicating parties. Over longer distances, it is common for the connection to be virtual, which means that although the two users can communicate only with each other, their signals and others are multiplexed, amplified, switched, scrambled, demultiplexed and so on in complex ways between the end points.
This contrasts with a dial-up connection which is only opened when one end requires it.