Star Trek: What Does NX Stand For?
Like many ships in the real world, fleet-allied ships in the "Star Trek” franchise often come with a registry code and prefix. In addition to the Starfleet prefix "USS," each ship has a unique designation code. Perhaps the best-known is the registry printed on the side of Captain James Kirk's Enterprise, as seen on "Star Trek: The Original Series" — NCC-1701. In a series where ship names tend to get reused, these codes can be especially useful for helping audiences keep track of which version of the Enterprise they're dealing with. But long before so much as a self-sealing stem bolt could be placed in the NCC-1701-A, the NCC-1701-B, the NCC-1701-C, or their successors, the ship's first iteration would roll out under the registry NX-01, as seen in "Star Trek: Enterprise."
While there is no clear in-canon explanation for what the letters "NX" themselves mean, the term seems to derive from Starfleet's NX Test Program. As former test pilot and eventual Enterprise Captain Jonathan Archer (Scott Bakula) recounted to T'Pol (Jolene Blalock) in the "Enterprise" episode "First Flight," this program was designed to break the warp 2 barrier. Other ships would eventually take this designation, which many fans theorize is used to designate prototypical ships.
NX ships are experimental
Despite the lack of an in-world explanation for the meaning behind "NX," many fans and "Star Trek" novelists have come to accept the meaning behind Starfleet's designation "NCC" as naval in origin, leading to speculation that "NX" could stand for "naval experiment." This makes sense since the letters are generally connected to experimental and prototypical technology.
As related in Archer's "First Flight" tale, there were only a few pilots involved with the original NX Test Program, including himself, A.G. Robinson (Keith Carradine), Duvall, and Gardner. The program was highly competitive, with each of the pilots involved vying for the first flight using the engine Archer's father had designed. The first test ship, the NX-Alpha, broke up shortly after hitting warp 2.2, with Archer and Robinson eventually exceeding 2.5 in the NX-Beta.
The first starship class developed using this engine was designated as "NX-class," with two of the most notable ships being the Enterprise NX-01 and Columbia NX-02; the latter is seen in the "Enterprise" episode "Affliction." This designation can be seen as late as the 24th century, with the prototypical transport ship the USS Bradbury NX-72307 that shows up in the "Star Trek: The Next Generation" episode "Ménage à Troi," among others.