a line that connects two or more extensions in a PBX telephone system.
a private telephone channel, leased from a telephone company, that connects two or more PBX systems.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Examples from the Web for tie line
Thus, the points a′b′ represent conjugate solutions, and the line a′b′ is a tie-line.The Phase Rule and Its Applications|Alexander Findlay
British Dictionary definitions for tie line
a telephone line between two private branch exchanges or private exchanges that may or may not pass through a main exchange
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012