A tripwire or trip line was any length of line that could be used as part of a booby trap, designed to trigger an alarm or simply trip someone to make them fall.
Kirk setting a basic tripwire
After Captain James T. Kirk refused to help Jojo Krako become the top boss on Sigma Iotia II in 2268, Kirk was "put on ice" in the cellar of Krako's headquarters. While captive, Kirk took the copper wire out of a vacuum tube radio and strung it between a desk and a table to create a tripwire. He then feigned an injury that lured a Iotian gangster into the room, where he tripped over the wire, before he was subsequently incapacitated by Kirk. (TOS: "A Piece of the Action")
An officer pulling a trip cord tight
After several Gelrakian boarding parties came aboard the USS Cerritos in 2380, they pursued and attacked several members of the Cerritos' crew, including one officer who lured two Gelrakians into the shuttlebay, where they were tripped by another officer who pulled a large cable trip cord. (LD: "Temporal Edict")