The concept derives from the notion that the original speakers of the Indo-European languages and their descendants up to the present day constitute a distinctive race or subrace of the Caucasian race.
The term Aryan has generally been used to describe the Proto-Indo-Iranian language root *arya which was the ethnonym the Indo-Iranians adopted to describe Aryans. Its cognate in Sanskrit is the word ?rya (Devan?gar?: ?), in origin an ethnic self-designation, in Classical Sanskrit meaning "honourable, respectable, noble". The Old Persian cognate ariya- (Old Persian cuneiform) is the ancestor of the modern name of Iran and ethnonym for the Iranian people.
The term Indo-Aryan is still commonly used to describe the Indic half of the Indo-Iranian languages, i.e., the family that includes Sanskrit and modern languages such as Hindi-Urdu, Bengali, Nepali, Punjabi, Gujarati, Romani, Kashmiri, Sinhala and Marathi.
In the 18th century, the most ancient known Indo-European languages were those of the ancient Indo-Iranians. The word Aryan was therefore adopted to refer not only to the Indo-Iranian peoples, but also to native Indo-European speakers as a whole, including the Romans, Greeks, and the Germanic peoples. It was soon recognised that Balts, Celts, and Slavs also belonged to the same group. It was argued that all of these languages originated from a common root - now known as Proto-Indo-European - spoken by an ancient people who were thought of as ancestors of the European, Iranian, and Indo-Aryan peoples.
In the context of 19th-century physical anthropology and scientific racism, the term "Aryan race" came to be misapplied to all people descended from the Proto-Indo-Europeans - a subgroup of the Europid or "Caucasian" race, in addition to the Indo-Iranians (who are the only people known to have used Arya as an endonym in ancient times). This usage was considered to include most modern inhabitants of Australasia, the Caucasus, Central Asia, Europe, Latin America, North America, Siberia, South Asia, Southern Africa, and West Asia. Such claims became increasingly common during the early 19th century, when it was commonly believed that the Aryans originated in the south-west Eurasian steppes (present-day Russia and Ukraine).
Max Müller is often identified as the first writer to mention an "Aryan race" in English. In his Lectures on the Science of Language (1861), Müller referred to Aryans as a "race of people". At the time, the term race had the meaning of "a group of tribes or peoples, an ethnic group". He occasionally used the term "Aryan race" afterwards,  but wrote in 1888 that "an ethnologist who speaks of Aryan race, Aryan blood, Aryan eyes and hair, is as great a sinner as a linguist who speaks of a dolichocephalic dictionary or a brachycephalic grammar";
While the "Aryan race" theory remained popular, particularly in Germany, some authors opposed it, in particular Otto Schrader, Rudolph von Jhering and the ethnologist Robert Hartmann (1831-1893), who proposed to ban the notion of "Aryan" from anthropology.
Müller's concept of Aryan was later construed to imply a biologically distinct sub-group of humanity, by writers such as Arthur de Gobineau, who argued that the Aryans represented a superior branch of humanity. Müller objected to the mixing of linguistics and anthropology. "These two sciences, the Science of Language and the Science of Man, cannot, at least for the present, be kept too much asunder; [...] I must repeat, what I have said many times before, it would be as wrong to speak of Aryan blood as of dolichocephalic grammar". He restated his opposition to this method in 1888 in his essay Biographies of words and the home of the Aryas.
By the late 19th century the steppe theory of Indo-European origins was challenged by a view that the Indo-Europeans originated in ancient Germany or Scandinavia - or at least that in those countries the original Indo-European ethnicity had been preserved. The word Aryan was consequently used even more restrictively - and even less in keeping with its Indo-Iranian origins - to mean "Germanic", "Nordic" or Northern Europeans. This implied division of Caucasoids into Aryans, Semites and Hamites was also based on linguistics, rather than based on physical anthropology; it paralleled an archaic tripartite division in anthropology between "Nordic", "Alpine" and "Mediterranean" races. The German origin of the Aryans was especially promoted by the archaeologist Gustaf Kossinna, who claimed that the Proto-Indo-European peoples were identical to the Corded Ware culture of Neolithic Germany. This idea was widely circulated in both intellectual and popular culture by the early twentieth century, and is reflected in the concept of "Corded-Nordics" in Carleton S. Coon's 1939 The Races of Europe.
This usage was common among knowledgeable authors writing in the late 19th and early 20th centuries. An example of this usage appears in The Outline of History, a bestselling 1920 work by H. G. Wells. In that influential volume, Wells used the term in the plural ("the Aryan peoples"), but he was a staunch opponent of the racist and politically motivated exploitation of the singular term ("the Aryan people") by earlier authors like Houston Stewart Chamberlain and was careful either to avoid the generic singular, though he did refer now and again in the singular to some specific "Aryan people" (e.g., the Scythians). In 1922, in A Short History of the World, Wells depicted a highly diverse group of various "Aryan peoples" learning "methods of civilization" and then, by means of different uncoordinated movements that Wells believed were part of a larger dialectical rhythm of conflict between settled civilizations and nomadic invaders that also encompassed Aegean and Mongol peoples inter alia, "subjugat[ing]" - "in form" but not in "ideas and methods" - "the whole ancient world, Semitic, Aegean and Egyptian alike".
In the 1944 edition of Rand McNally's World Atlas, the Aryan race is depicted as one of the ten major racial groupings of mankind. The science fiction author Poul Anderson, an anti-racist libertarian of Scandinavian ancestry, in his many works, consistently used the term Aryan as a synonym for "Indo-Europeans".
The use of "Aryan" as a synonym for Indo -European may occasionally appear in material that is based on historic scholarship. Thus, a 1989 article in Scientific American, Colin Renfrew uses the term "Aryan" as a synonym for "Indo-European".