The truest definition for overpowered is essentially being invincible, unable to lose whether by a great defense or great offense or both. When you have zero chance to fail, even after basic human error play their part, you're overpowered.

This works the other way too, AV back then, PW, ADL, to name a few, are overpowered. The price we paid for the FFXI end game style. Without a long term tier system of upgrades, SE often fell back on overpowered monsters to create challenge since players did not have a continuously upgraded gear system in place to be able to defeat the next big bad, like EQ.