A barbarian is a person or tribe of people that is perceived to be primitive, savage, and warlike. A "barbarian" may also be an individual reference to an aggressive, brutal, cruel, and insensitive person, particularly one who is also dim-witted, while cultures, customs and practices adopted by peoples and countries perceived to be primitive may be referred to as "barbaric".

The term originates from the Ancient Greek: βάρβαρος (barbaros; pl. βάρβαροι barbaroi). In ancient Greece, the Greeks used the term not only for those who did not speak Greek and follow classical Greek customs, but also for Greek populations on the fringe of the Greek world with peculiar dialects. In ancient Rome, the Romans adapted and applied the term to tribal non-Romans such as the Xiongnu, Mongols, Germans, Celts, Iberians, Helvetii, Thracians, Illyrians, and Sarmatians. In the early modern period and sometimes later, the Byzantine Greeks used it for the Turks in a clearly pejorative manner.

The Greek word was borrowed into Arabic as well, under the form بربر (barbar), and used as an exonym by the Arab invaders to refer to the indigenous peoples of North Africa, known in English as Amazigh or Berbers, with the latter thereby being a cognate of the word "barbarian".

Historically, the term barbarian has seen widespread use. Many peoples have dismissed alien cultures and even rival civilizations, because they were unrecognizably strange. For instance, the nomadic Turkic peoples north of the Black Sea, including the Pechenegs and the Kipchaks, were called barbarians by the Byzantines.

The ancient Greek name βάρβαρος (bárbaros) 'barbarian' was an antonym for πολίτης (politēs) 'citizen', from πόλις (polis) 'city'. The earliest attested form of the word is the Mycenaean Greek 𐀞𐀞𐀫, pa-pa-ro, written in Linear B syllabic script.

The Greeks used the term barbarian for all non-Greek-speaking people, including the Egyptians, Persians, Medes, and Phoenicians, emphasizing their otherness. According to Greek writers, this was because the language they spoke sounded to Greeks like gibberish represented by the sounds "bar..bar..;" the alleged root of the word bárbaros, which is an echomimetic or onomatopoeic word. In various occasions, the term was also used by Greeks, especially the Athenians, to deride other Greek tribes and states (such as Epirotes, Eleans, Boeotians, and Aeolic-speakers) and also fellow Athenians in a pejorative and politically motivated manner. The term also carried a cultural dimension to its dual meaning. The verb βαρβαρίζω (barbarízō) in ancient Greek meant to behave or talk like a barbarian, or to hold with the barbarians.

Plato (Statesman 262de) rejected the Greek–barbarian dichotomy as a logical absurdity on just such grounds: dividing the world into Greeks and non-Greeks told one nothing about the second group. Yet Plato used the term barbarian frequently in his seventh letter. In Homer's works, the term appeared only once (Iliad 2.867), in the form βαρβαρόφωνος (barbarophonos, ‘of incomprehensible speech’), used of the Carians fighting for Troy during the Trojan War. In general, the concept of barbaros did not figure largely in archaic literature before the 5th century BC. It has been suggested that the ‘barbarophonoi’ in the Iliad signifies not those who spoke a non-Greek language but simply those who spoke Greek badly.

A change occurred in the connotations of the word after the Greco-Persian Wars in the first half of the 5th century BC. Here a hasty coalition of Greeks defeated the vast Persian Empire. Indeed, in the Greek of this period 'barbarian' is often used expressly to refer to Persians, who were enemies of the Greeks in this war.