that is (to say)

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said when you want to give further details or be more exact about something

那就是说;换句话说

I'll meet you in the city, that is, I will if the trains are running.

我们城里见，那就是说，只要火车还开我就会见到你。

That would mean voting no, that is to say, using the veto.

那就意味着投反对票，也就是说，行使否决权。