Earthling, Terran, and Gaian are terms commonly used in science fiction to identify humans as opposed to extraterrestrials. The literary effect aimed for is a distancing effect, inviting the readers to contemplate their own species as it might be seen from an external point of view. Especially in 1950s science fiction, use of the terms is a conscious reversal of common assumptions of anthropocentrism or human exceptionalism, and may be an example of a demonym.
Historically the term referred to a mortal inhabitant of the Earth as opposed to spiritual or divine entities.
The derivation from the noun earth by means of the suffix -ling is already seen in Old English eyrþling, in the meaning "ploughman". The sense of "inhabitant of earth" is first attested in 1593. Its use in science fiction dates to 1949, in Red Planet by Robert A. Heinlein.
In Early Modern English, the word was used with the intention of contrasting "earth" with "heaven", and so presenting man as an inhabitant of the sublunary sphere, as opposed to heavenly creatures or deities.
Its modern use in science fiction literature contrasts "Earth" (the planet) with outer space or hypothetical other planets with sapient life. The term was often used in 1950s science fiction film and novels by aliens to express a disdainful or patronising tone towards creatures from Earth. The meaning "creature from planet Earth" in the context of space travel may be extended to non-human species, as in "Russia fetes dog Laika, first earthling in space".