In a similar way to
つまり,
即（すなわ）ち is another grammar pattern in Japanese that is often translated as ‘in other words’. It may also be translated as ‘namely’, or ‘that is to say’. While
即（すなわ）ち is a 副詞（ふくし） (adverb), it is often used as a 接続詞（せつぞくし） (conjunction), linking two sentences via some sort of reasoning.
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この人（ひと）は私（わたし）の妻（つま）のお母（かあ）さんです。すなわち、義理（ぎり）の母（はは）です。
This person is my wife's mother. In other words, my mother in law.
私（わたし）はゲームしてお金（かね）を稼（かせ）いでいます。すなわち、プロゲーマーです。
I get paid for playing video games. In other words, I am a professional gamer.
The kanji used in
即（すなわ）ち simply means ‘in concordance’. This shows that the nuance associated with
即（すなわ）ち is that of ‘agreeance’ or ‘correspondence’ to something else, while
つまり is purely that of ‘summarisation’.
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この車（くるま）はもう動（うご）かないです。すなわち廃車（はいしゃ）です。
This car does not run anymore. In other words, it is a decommissioned car.
彼（かれ）は僕（ぼく）の父（ちち）の弟（おとうと）です。つまり叔父（おじ）です。
He is my father's brother. In other words, he is my uncle.