According to the [Science Fiction Citations](http://www.jessesword.com/sf/) site, the earliest known use of ["Terran" as a noun](http://www.jessesword.com/sf/view/127) meaning "an inhabitant of the planet Earth" (aka Earthian, Earthie, Earthling, Earthman, Tellurian, Terrestrial, Terrestrian, etc.) was in [*Astounding Science-Fiction*, May 1946](http://www.isfdb.org/cgi-bin/pl.cgi?57593+c), in part 3 of the serialization of [George O. Smith](https://en.wikipedia.org/wiki/George_O._Smith)'s novel [*Pattern for Conquest*](http://www.isfdb.org/cgi-bin/title.cgi?21772):

> *The mission, not entirely understood by the Terrans, consists of destroying a machine sent forth by the Loard-vogh, a race that is conquering the Galaxy on a twenty-thousand-year program. This machine restricts mental activity through a vast area, thus permitting the Loard-vogh to advance without difficulty. Communication between the Little People of Tlembo and Terrans is also restricted by the machine, and so the true nature of the mission is not really known.*

The earliest citation for ["Terran" as an adjective](http://www.jessesword.com/sf/view/126) is from the 1881 novel [*Three Hundred Years Hence*](http://www.isfdb.org/cgi-bin/title.cgi?1010493) by [William Delisle Hay](http://www.isfdb.org/cgi-bin/ea.cgi?124608).

The earliest citation for ["Terra"](http://www.jessesword.com/sf/view/123) as a name for the planet Earth is from an 1871 lecture "Science & Revelation" by [Robert Payne Smith](https://en.wikipedia.org/wiki/Robert_Payne_Smith):

> Now, let us suppose ourselves philosophers come, we will say, form the planet Jupiter, on a mission intrusted to us by the Jovians, to examine and report upon the nature of the creatures which people the four inferior planets, Terra, Venus, Mercury, and Mars.