Final answer:
In Greek and Roman societies, the term barbarian was used to describe foreigners whose languages sounded like incomprehensible speech ('bar-bar-bar') to them and who were considered less civilized. Romans considered non-Roman cultures, such as the Thracians, Germans, and Celts, to be barbarians, often regarding them as inferior and either depicted as adversaries in war or subjects of the empire.
Step-by-step explanation:
The criteria for defining a "barbarian" in the Greek and Roman worlds centered around the concept of cultural and linguistic differences. To Greeks and Romans, the term barbarian derived from the Greek 'barbaros' which meant 'foreigner'. This label stemmed from the Greeks hearing non-Greek languages as unintelligible speech ('bar-bar-bar'), which subsequently became a pejorative term suggesting that such cultures were less civilized. In the Roman context, barbarians were often those outside the empire, such as the Thracians, Germans, and Celts, who did not share Roman ideals and practices, and were depicted in Roman texts as inferior or as enemies to be subdued or enslaved.
Romans had complex relations with these peoples, at times engaging in trade, recruiting them as soldiers, fighting against them, or portraying them in a negative light as wild or morally inferior. Yet, the Romans did not always accurately distinguish between different 'barbarian' groups, and commonly used blanket terms such as 'Goths' or 'Scythians' for identifying various tribes. The differing attitudes towards barbarians in the Greek and Roman civilizations reflect their own cultural biases and the dynamics of power.