A phase coil was a component of a transporter system.
A parametric scanner was typically used for aligning phase coils. (ENT: "Singularity")
In November 2154, the primary phase coil of the personnel transporter of the Enterprise NX-01 failed, disabling the transporter system. (ENT: "Babel One")
Before 2367, Geordi La Forge had upgraded the phase coils of the USS Enterprise-D to fifty-five field densities. (TNG: "Galaxy's Child")
It is unclear if this is transporter component is that same as the similarily named transporter component known as the phase transition coil.