1A conversational connective used to close off a conversation or introduce what comes next (like "well," "now," or "right").
それでは、そろそろ失礼します。
Well then, I should be heading off soon.
それでは、次の議題に入りましょう。
Right, let's move on to the next agenda item.
それでは、自己紹介から始めましょう。
Now then, let's start with self-introductions.
2A conjunction meaning "so" or "then" — draws a conclusion or a course of action from what was just said.
雨が降ってきた。それでは試合は中止だ。
It's started raining. In that case, the match is cancelled.
君が忙しいなら、それでは私が代わりに行くよ。
If you're busy, then I'll go instead.
彼が来られない。それでは私たちだけで始めよう。
He can't come. So let's start without him.
3Meaning "if that is the case" or "in that situation" — used to spell out what follows from a stated condition.
締め切りが延びたのなら、それでは準備にもっと時間が取れるね。
If the deadline was pushed back, then that means we get more time to prepare.
会場が使えないのなら、それでは別の場所を探そう。
If we can't use the venue, then let's look for another place.
4An interjection used when saying goodbye or taking one's leave (equivalent to "well then" / "bye then").
A：もう行きます。B：それでは、また。
A: I'm heading out now. B: Bye then, see you.
それでは、失礼します。
Well then, excuse me — I'll be off.