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Meaning of tie line

tie' line"

Pronunciation: [key]
— Telecommunications. Telecommunications.
  1. a line that connects two or more extensions in a PBX telephone system.
  2. a private telephone channel, leased from a telephone company, that connects two or more PBX systems.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.