NX
From Memory Alpha, the free Star Trek reference.
The USS Defiant (NX-74205) in 2373
NX is a starship registry number prefix in use by both the Earth and Federation Starfleets.
In the Earth Starfleet it was used to represent the NX class starships. (ENT: "Fortunate Son")
In the Federation Starfleet, they came to be used by experimental starship designs that typically became the first starship (or prototype) of their class. (Star Trek III: The Search for Spock)
The NX registry was also in use in the mirror universe to represent the NX class of starships. (ENT: "In a Mirror, Darkly")
