Earth type was a generic term used to describe something of Earth origin.
It could also be a means of comparing another planet to Earth itself, so as to also describe a class M planet.
During the Romulan Neutral Zone Incursion of 2266, Doctor Leonard McCoy pointed out the Captain James T. Kirk that "In this galaxy, there's a mathematical probability of three million Earth-type planets. And in all of the universe, three million million galaxies like this. And in all of that, and perhaps more, only one of each of us. Don't destroy the one named Kirk." (TOS: "Balance of Terror")
After Captain Kirk, Hikaru Sulu, Dr. McCoy, and D'Amato became stranded on the artificial Kalandan outpost planet in 2269, they initially found no evidence of rainfall. Kirk then pointed out that the planet had "Earth-type vegetation", which should have otherwise indicated the presence of water. (TOS: "By Any Other Name")
Later that year, Kirk and Spock were invited down to the surface of Excalbia. When they invited for comments from the senior staff, McCoy incredulously pointed out the simple fact that moments before, the planet's surface was shown be entirely molten lava, then "suddenly, miraculously, we see a small spot of Earth-type environment down there." (TOS: "The Savage Curtain")