Star Trek spent decades using M-class as shorthand for Earth-like worlds before the franchise finally explained what the “M” in the designation meant. In the Enterprise episode “Strange New World,” the “M” was spelled out as “Minshara Class,” giving a name to a term that had floated through the franchise since The Original Series.

The reveal matters because M-class has been part of the show’s basic language for generations. The Original Series introduced Earth-like planets as M-class worlds, and the term was repeated in The Next Generation, Deep Space Nine and Voyager. In “Balance of Terror,” the franchise also established that there are likely around three million Earth-like planets in the Milky Way galaxy, while The Next Generation later described four levels of M-class planets.

“Strange New World” was also the first episode in the franchise to make clear that the Vulcans were behind the terminology. At the time, Vulcan explorers gathered information from probes rather than planetary scans, and they used the term “Minshara-Class” for Earth-like planets. Captain Archer’s Enterprise became the first Starfleet vessel to adopt that language, and within three years it had become standard fleetwide.

That sequence gives the term a history that had been hidden in plain sight for decades. The designation was already familiar to viewers long before its meaning was explained, which is why the detail lands as both a retcon and a cleanup of the franchise’s internal vocabulary. The article says the Vulcans originated the terminology and Starfleet later adopted it.

The answer, then, is simple: the “M” in M-class stands for “Minshara Class,” and “Minshara-Class” is the Vulcan term that eventually spread through Starfleet. What had once been only a label now has a source, a transmission path and a place in the franchise’s own history.