This is only partially accurate. The Romans and Persian, for example, did not consider the other, barbarians. Same between the Greeks/Egyptians/Carthaginians/Jews/etc...
What defined a "barbarian" group was that they we undeveloped scattered tribes - without central political power. They usually lacked written codified laws, formal political assembly, ambassadors, etc...
The Gauls, the Celts, the Germanic tribes, the Turkic tribes, etc... They all were considered "barbarians" because of their tribal community focused political structure. Few centralized buildings existed, lower literacy rates, etc...
These differences were very apparent when a soldier would leave from Rome or Constantinople, and then find himself conquering a Gaulic settlement where the biggest buildings were still made of wood.
Of course, as the centuries went on, these tribal communities became more and more centralize and organized until those differences disappeared.
The Greeks used the term barbarian for all non-Greek-speaking peoples, 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 βάρβαρος, 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, Macedonians, Boeotians and Aeolic-speakers) and also fellow Athenians in a pejorative and politically motivated manner.
What defined a "barbarian" group was that they we undeveloped scattered tribes - without central political power. They usually lacked written codified laws, formal political assembly, ambassadors, etc...
The Gauls, the Celts, the Germanic tribes, the Turkic tribes, etc... They all were considered "barbarians" because of their tribal community focused political structure. Few centralized buildings existed, lower literacy rates, etc...
These differences were very apparent when a soldier would leave from Rome or Constantinople, and then find himself conquering a Gaulic settlement where the biggest buildings were still made of wood.
Of course, as the centuries went on, these tribal communities became more and more centralize and organized until those differences disappeared.