Intransitive balance
Also said as Rock paper scissors, Counter system, Weapon triangle
A loop of counters where A beats B, B beats C and C beats A, so there is no best choice, only a best choice against what the other side is doing.
It is the cheapest way to get balance: no number needs to be perfect, because everything already has an answer. It moves the skill to reading your opponent. Too strong and matches are settled at the pick screen.
Say it
“Let archers beat infantry 2 to 1, infantry beat cavalry 2 to 1, and cavalry beat archers 2 to 1, and scout the enemy before each battle so the choice can be informed.”
Where you’ve seen it
The Fire Emblem series uses a weapon triangle: swords beat axes, axes beat lances, lances beat swords. Pokémon's types are a much bigger version of the same idea.
Not to be confused with fair
Everything being evenly matched is one kind of balance. Intransitive balance is the opposite: every match-up is lopsided, and the whole set evens out.