A plasma coil is a component of a starship's engines.
The D-12 class Klingon Bird-of-Prey was retired from service as its plasma coils were defective. (Star Trek Generations)
In 2151, shortly before the maiden voyage of Enterprise NX-01, valve sealant was beamed up to the ship instead of plasma coils, to Lieutenant Malcolm Reed's chagrin. (ENT: "Broken Bow")
In 2153, magnetic containment on Enterprise was fluctuating, due to the Sphere Builder test subject trying to destroy Enterprise by destablising the magentic seals on the warp core. Malcolm Reed and Major J. Hayes were able to defeat the Sphere Builder by reversing the polarity of the plasma coils to trigger a feedback pulse. (ENT: "Harbinger")
While visiting Deep Space 9 in 2369, Geordi La Forge was interested in purchasing a 21st century Ktarian plasma coil that he found in a shop on the Promenade in near perfect condition. (TNG: "Birthright, Part I")
In 2370, Data initialized the plasma induction coils on an infusion unit while in a cave on Atrea IV trying to stop the cooling of the planet's molten core. (TNG: "Inheritance")
In an unknown, future timeline, the USS Enterprise-D took a hit to its port plasma coil from the Klingons, which damaged the ship's cloaking device. Admiral William T. Riker had the Enterprise taken back to Federation space at warp 13, without the cloak. (TNG: "All Good Things...")
Loose plasma coils create large amounts of radiation. A Bajoran transport docking at Deep Space 9 in 2371 had a loose plasma coil, preventing the station from beaming off wounded passengers. (DS9: "Life Support")
Plasma coils are also highly deadly, as an overload often causes computer consoles to explode in a shower of sparks, sometimes killing crewmembers. (citation needed • edit)
In 2372, Bejal Otner reported that the plasma coil interlocks were activated when the USS Defiant was conducting tests on Dr. Lenara Kahn's theory of artificial wormholes, which the Federation was interested in investigating. (DS9: "Rejoined")
Fluctuations in the plasma coils can cause a power surge in the warp core. When this occurs, the ship is typically required to take the core offline. (DS9: "Our Man Bashir")