The earliest Sci-fi reference I've found to "Terran" or "Terrans" (in a scifi context) is from a February 1945 edition of Astounding Science Fiction where the word is used twice, firstly in a story called called Lilies of LifeLilies of Life by Malcolm Jameson where it is used to describe terrestrial plant life.
Maxwell studied the plants with interest, but saw little to distinguish them from the Terran variety except their great size and yellow color.
And then again in a separate story called NomadNomad by George O. Smith under his pseudonym, Wesley Long.
The nomad world that wanted no part of Sol’s warfare and strife; killing and death. They knew — they knew from the things he said — that Terra was a planet of self-aggrandizement and that Terrans were proud, haughty, and belligerent.
Note that the word Terran is used as a direct contrast alongside Venusians, Martians and Ertinians (after a fictional planet called Ertene that's entered the solar system).