virtual path identifier This is an 8-bit field in the header of an Asynchronous Transfer Mode (ATM) cell. The VPI, together with the virtual channel identifier (VCI), is used to identify the next destination of a cell as it passes through a series of ATM switches on its way to its destination. ATM switches use the VPI/VCI fields to identify the next virtual channel link (VCL) that a cell needs to transit on its way to its final destination. The function of the VPI is similar to that of the data-link connection identifier (DLCI) in Frame Relay. Compare with DLCI. See also VCI and VCL.