Can we discuss terminology?
Overpowered: Over powered things are things that have too much UTILITY OR have too high of NUMBERS.
How Overpowered Champions are Addressed: Normally with loss of NUMBERS or loss of UTILITY.
Example of when these moments occurred:
90%AP Q or
Healing.
Underpowered: Under powered things are things that don't have enough UTILITY or too low of DAMAGE output.
How Underpowered Champions are Addressed: Normally with massive buffs to NUMBERS or unreasonable cramming of UTILITY.
Example of when these moments occurred: Fighters for a long period of time prior to Conqueror. Such as 
Overperforming: Over performing are things that are doing well or better because of outside influences.
How Overperforming Champions are Addressed: Normally with loss of minor NUMBERS or UTILITY.
Example of when these moments occurred: Enchanter Supports during
meta. ADC's were picked to function well with Ardent. [Yes ADC's were strong but ADC's also were Overperforming due to Ardent]
Underperforming: Under performing are things that are doing poorly or worse because of outside influences.
How Underperforming Champions are Addressed: Normally with minor buffs to NUMBERS or UTILITY
Example of when these moments occurred:
post Mid Season Mage Update.
Balanced: If talking statistically normally reaching or hovering around 50% in both win and loss with reasonable pick rate [Normally not exceeding 7%].
When talking about gameplay.
-Has counterplay.
-Has clear weakness.
-Has clear strength.
-Has damage = how good/bad its laning phase was.
--Bad Laning Phase = Great late game damage.
--Good Laning Phase = lack of impact late game.
-Is not Overpowered, Overperforming, Underpowered, or Underperforming.
-Balance In All Things, Except for League. [Please tell me someone gets this joke.]