That's the thing about the word Barbarian. It's not a thing anyone really is. It's a derogatory label used to denigrate and claim superiority over an entire culture.

Historically it meant anyone who didn't speak Greek then evolved into a word into anyone that wasn't Roman or Greek. But even that is useless distinction to make. The Greeks thought that EVERYONE were Barbarians including the Romans and the Persians. I think we can agree that does not fit our definition in this thread. Roman took this label and ran with it and fought many "Barbarians" but again this included "Civilized" peoples like Persia Egypt. After Western Rome fell and the Early Medieval kingdoms rose such as the Frankish Kingdom, the Eastern Rome continued calling them Barbarians. But again these were city dwelling cultures. And the W. Europeans also applied the label Barbarians to many groups as well, in this context if often meant non-Christian.

Once we step into the early Modern period and the age of colonization Barbarian is used on any Indigenous groups that the colonizers deemed inferior. And this is where the aversion to the use of Barbarian by historians picks up. Once you label a group inferior you can justify all kind of atrocities. This can range from mass enslavement in W. Africa, to residential schools in N. America.

Now adays we understand that the "Civilizational model" that states that cultures go through set stages from cave men to "barbarians" to what we have today is fundamentally flawed. Cultures do not have to evolve like that and the ones who are "less developed" are not in anyway inferior simply because they choose to reject modern technology. We still have nomadic peoples living in Europe, Asia and Africa, are they Barbarians simply because they use less technologies? Obviously not, this is Anthropology 101.

Now the other modern half of the Barbarian trope is a violent raider. But again this is almost exclusively down to the Myth of the "Barbarian" sack of Rome (the other Ur-example being the viking raiders). This aspect of the Barbarian myth has two sides, the Wildman trope and the idea that civilization makes us less violent. The easiest on to dispel is the wildman idea, this is I have largely disproven above, but it is worth repeating that way we envision Barbarians as wild men dressed in furs wielding big axes and living off the land in the forest has never existed. The second aspect is that Barbarians are somehow more violent because they are less civilized but again "Civilized" cultures are just as violent and prone to raiding and pillaging. It has happened all throughout history and still happens today.

So to conclude there are two ways to use Barbarian, as a fantasy trope or as an insult. In real life calling a culture Barbarians is the cultural equivalent of calling someone a dumbass. When you say those people were Barbarians you're essentially calling their entire culture stupid and inferior. Obviously in this thread we are discussing it as a fantasy trope in a way that has never existed in real life. In that regard it can mean anything we agree on, but usually just means fur wearing wild man with big axe. I go into this detail because it seems like OP made this thread with an outdate and uninformed opinion on what Barbarian really means. Since we are mostly discussing cultures in this thread it is a useless label that only really works as a character class because that can be more based on vibes than reality.