ということは means "that means", "so": it takes a fact and leads into what follows from it. It goes after a plain-form sentence, or stands at the start of a sentence and points back at what was just said. The conclusion usually ends in , , or .
、。
The light is on, so someone must be there.
、。
If he isn't coming, that means there's nobody but us to do it.
、。
Nobody objected, which means everyone agreed.
Checking you have understood
At the start of a reply, ということは draws the conclusion from what the other person said, and a question checks that it is right.
あAきごう:。bBきごう:、きごう?
A: I'm being transferred to Tokyo next month. B: So you're moving?
きごう?、きごう?
You failed? So you'll be studying another year?
In casual speech it shortens to .
、。
Tomorrow's a day off, so I can stay up late tonight.
ということは or つまり
restates the same thing in other words. ということは moves on to something new that follows from it.
、。
He is my mother's younger brother – in other words, my uncle.
、。
The shop is shut, so today must be its regular day off.