For the purpose of this thread, balance would mean that a party’s overall performance is not impacted by what classes compose that party. A party of different character classes should have, on average, just as easy a time at an adventure than a different party of different classes.
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To me, balance means that the DM doesn’t need to worry about what class his party brings to the table, nor do they need to micro manage their resources (“Do you have enough guano left for that spell?”) and they can just focus on making the game fun and memorable. The DM should be confident that an adventure for four characters of level 5 to 8 will work for their group without any mechanical overhaul on their part as long as the group fits the presented criteria. Maybe the party will have to go at it in an unorthodox way, but they’ll have a shot at victory regardless.