Phase lock was an engineering term used to describe the proper alignment of a vessel's warp fields.

In 2266, as the USS Enterprise was spiralling down towards planet Psi 2000, chief engineer Montgomery Scott was forced to develop a brand new startup routine for the ship's engines, as they had been deactivated and would have taken 30 minutes to start again without a proper phase lock.

In the 23rd century, Starfleet vessels were required to phase lock their warp fields within three percent or they would become unstable. By 2369, however, Starfleet began to utilize multiphase autocontainment fields, which were designed to operate above three percent, a fact which surprised Captain Scott when he came aboard the USS Enterprise-D. (TOS: "The Naked Time"; TNG: "Relics")

In 2373, Neelix suggested that B'Elanna Torres phase lock the plasma injectors to the dilithium matrix as the plasma flow in the manifold was constricted. She'd already done so. (VOY: "Fair Trade")

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