"Barbarian" is one of many words from all over the world and throughout all periods of history that refer to "others" or "those who can't speak our language". The meaning of "barbarian" (βάρβαρος) changes somewhat through Greek history, but early on, it referred specifically to people who did not speak Greek - "bar-bar-bar" is how the Greeks heard non-Greek language; think of the Dave Chapelle bit "Ching chang chong I can't understand you!" referring to Mandarin, or people trying to imitate the sounds of any language they don't understand. An Italian song from the early 70's called Prisencolinensinainciusol by Adriano Celentano provides an example for English. It did not refer to any non-Greek people specifically, but it certainly carried (at first, neutral) connotations of being a non-Greek person. By extension, it also came to mean someone speaking broken Greek, or Greek in a thick foreign accent ("barbarophonos").

Bearing that in mind, another important element of the definition had to do with slavery in Greece. In most cities, temporary servitude of Greeks was an accepted punishment for non-payment of debts. In Athens it was banned under Solon, with the argument that all Greeks are equal and free. To replace this lost labor (especially in the dangerous mining industry) they increased slavery of non-Greeks. These "barbarians" could usually not easily communicate or understand orders (and there was no organized effort to teach every slave proper Greek), and because language and intelligence were (and are) seen as intimately connected, they were seen as dim-witted and inferior people. What was originally a purely linguistic descriptor eventually expanded as time went on, and the "barbarians" became ever more feeble and deserving of their fate, and slavery increasingly justified.

This negative view was not universal however, as Xenophon's Cyropaedia shows, where he speaks positively about the history of Cyrus the Great. Alexander the Great (who himself was considered a barbarian by some), during his conquests even adopted "barbarian" customs much to the chagrin of his cohorts, and encouraged his soldiers to take barbarian wives.

The Romans, for their part, inherited the word largely intact as it was during the Hellenistic period, but substituting the Greek for "non-Latin", or "non-Roman". The Germanic peoples were certainly barbarians, but it was never used to refer specifically to the Germanic peoples, as the Gauls, Huns, and Carthaginians were equally "barbaric". In fact, the term "Berber" comes directly from Barbarian. In Rome, as in Greece earlier, the negative connotations of "barbarian" were not universal, especially late in the Imperial period when multinationalism and multiculturalism became more common.