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 Earthling, Earthian, Earthman, Tellurian, 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.*