In planetary classification, a class M planet or planetoid was characterized by an abundance of surface water and an atmosphere high in nitrogen, with oxygen and trace elements. M-class planets were also termed Minshara class by Vulcans during the mid-22nd century. (ENT: "Strange New World") Life was often abundant on class M planets; they contained extensive vegetation, animal life, and often had native humanoids.
The terminology "Earth-type" was also used to describe M planets. By the 23rd century it was theorized that there was a mathematical probability of three million Earth-type planets in the Milky Way Galaxy alone. (TOS: "Balance of Terror")
Spock called the planet from TOS: "Return to Tomorrow" class M despite the atmosphere being ripped away, suggesting that class M goes beyond being able to support life. Alternately, he could have been saying that the planet prior to losing its atmosphere would have been categorized as class M.
According to the Star Trek: Star Charts, class M planets have ages that range from three to ten billion years and a diameter between 10,000 and 15,000 kilometers. They are located within the ecosphere of a star system.