A conjunction meaning 'if that's the case' or 'in that case' — used to introduce a suggestion, decision, or response based on the preceding statement.
もう時間がない。それなら早めに出発しよう。
There's not much time. In that case, let's leave early.
彼は来られない。それなら私が代わりに行くよ。
He can't come. If so, I'll go instead.
その店は高い。それなら他の店を探そう。
That shop is expensive. In that case, let's look for another shop.
忙しい？それならまた今度にしよう。
Are you busy? If so, let's do it another time.