noun
Used in games or contests when the result is a tie; common in rock-paper-scissors.
じゃんけんであいこだった。
It was a draw in rock-paper-scissors.
試合はあいこに終わった。
The match ended in a tie.
noun
Used when two parties are even after settling a debt, score, or exchange; often in the phrase あいこにする.
これで貸し借りはあいこだ。
Now we're even on debts.
おごってもらったから、今度は私がおごってあいこにしよう。
You treated me, so next time I'll treat you and we'll call it even.
Derived from the phrase 相子 (あいこ), literally 'mutual child', indicating a state of equality. The exact historical derivation is uncertain, but it has been used since the Edo period.