と言(い)う事(こと)はThat means, That is to say

ということ turns phrases into nouns and is often used to clarify what was said. When followed by は as ということは, it marks the phrase as a topic for further explanation or comment. ということは can also begin a sentence to restate or interpret what came before, like 'So what you're saying is (A)' in English.

というのは and ということは are similar in that both take a phrase and turn it into a topic for further explanation. While sometimes interchangeable, というのは is typically followed by a definition or reason. ということは is primarily used to rephrase, clarify, or draw a conclusion from the previous statement.

These two expressions are similar in that both can be used when offering or seeking explanations. ことなの is used at the end of a phrase or sentence, often to clarify a particular point, e.g. 今日は来ないということなの？ ('So does that mean he's not coming today?'), while ということは begins a sentence and sets up a conclusion based on what was just said, e.g. ということは、今日は来ないんだ ('So that means he's not coming today').

つまり and ということは both rephrase earlier information, but differ in usage. つまり restates or explains something just said, e.g. つまりできないってこと？ ('So you're saying you can't do it?'). ということは introduces a logical inference or consequence, e.g. ということは、試合は中止だね ('So that means the game is canceled').

即ち and ということは both rephrase information, but differ in tone and function. 即ち is formal and used to define or clarify precisely, often in writing, e.g. 日本の首都、即ち東京 ('The capital of Japan—namely, Tokyo'). ということは is conversational and introduces a conclusion, e.g. 明日は休み？ということは、今日は早く帰れるかもね ('Tomorrow's a holiday? So that means we might get to leave early today').