Balance -
A game that works in such a way that everyone (including the GM!) has fun.
If we're talking about mechanical imbalance? That's when everyone in the party has fun contributing to a combat* meaningfully. If one person is overly powerful and overshadows others, it makes the others not have fun. On the reverse side, if someone is grossly underpowered compard to the rest of the group, then they likewise don't have fun in most cases.
* I'd say more about exploration / social pillars balancing out, but let me be honest. Too much of the game is devoted to combat, and there are entire games that go without any dice rolls when it comes to social stuff; most of the time, its all solved with a single die roll when it does happen. There's no meaningful mechanical complexity here. The three pillars themselves aren't balanced, where combat overshadows the other two.
A game that works in such a way that everyone (including the GM!) has fun.
If we're talking about mechanical imbalance? That's when everyone in the party has fun contributing to a combat* meaningfully. If one person is overly powerful and overshadows others, it makes the others not have fun. On the reverse side, if someone is grossly underpowered compard to the rest of the group, then they likewise don't have fun in most cases.
* I'd say more about exploration / social pillars balancing out, but let me be honest. Too much of the game is devoted to combat, and there are entire games that go without any dice rolls when it comes to social stuff; most of the time, its all solved with a single die roll when it does happen. There's no meaningful mechanical complexity here. The three pillars themselves aren't balanced, where combat overshadows the other two.