Martian

A Martian was an inhabitant of the planet Mars. Unlike the case with Venus, where there was a distinct, native species known as the Venusians, it was unclear that the term Martian referred to any one sentient species. Martian was largely an equivalent term to Earthling as once employed by the Eleventh Doctor (DW: The Hungry Earth) — a term that applied to more than one species on the planet Mars.

Native Martians
Several native Martian creatures, such as the Ice Warriors, evolved on Mars, (DW: The Ice Warriors) as did many other Martian lifeforms, such as Martian fungi. (DW: The Seeds of Death) There was also a native virus present on Mars. (DW: The Waters of Mars)

Human Martians
By the 22nd century, Mars had been colonised by humans, who pushed the native Ice Warriors into poverty. (PDA: Fear Itself) Once humanity had made Earth itself uninhabitable, the Usurians saw to it that the Humans could move to Mars. (DW: The Sun Makers)

Many humans considered themselves "Martian" after colonisation, as there came a point when humans were regularly being born there. Vel Karneen, commander of Space Army, self-identified as a Martian, for instance (DAN: The Secret of the Mountain) — as did the second-in-command of the ADF, Reb Shavron. (DAN: Terror Task Force)
 * For a more detailed history of human colonisation of Mars, see the separate article on Mars.

As colloquialism
Prior to colonisation, many humans simply interchanged the word Martian for the term alien — much like "flying saucer" was used as a generic term for alien spacecraft. For example, Donna Noble referred to the Tenth Doctor as a Martian before understanding he was, in fact, a Time Lord. (DW: The Runaway Bride)

Misidentified Martians
Ostensibly because of this penchant to imagine Mars as a likely source of alien life, some humans occasionally misidentified other species for Martians, just as Donna had done with the Tenth Doctor.
 * Daniel Llewellyn thought that the Sycorax were from Mars until Major Richard Blake told him that Martians looked completely different. (DW: The Christmas Invasion)
 * Patrick also initially thought that the Centuripede came from Mars to read other people's mail. (K9TV: The Last Oak Tree)