すなわち

1. 1That is to say, in other words: restates or clarifies the essence of the preceding statement.Ex. 1
2. 2Namely, which means: introduces an equivalent statement or a conclusion that follows logically from what precedes, often in definitional or logical writing.Ex. 2
3. 3Namely: links two nouns, identifying the first more precisely with the second (e.g. 日本の首都、すなわち東京).Ex. 3

すなわち belongs to formal written Japanese and is typical of essays, textbooks, legal or academic writing, and formal explanations. It signals that what follows is equivalent to the preceding content, either as a restatement or as a conclusion that amounts to the same thing. The more common つまり covers similar ground in both speech and writing.

• 1.
  つまりN3つまり can also rephrase a noun, restate what came before, or sum up a conclusion, and its meaning is basically the same as すなわち — in formal writing the two are often interchangeable. However, つまり works in both speech and writing, so in everyday conversation you should say つまり.
• 2.
  要するにN2要するに is neutral in register and common in speech; it focuses on distilling long content down to its key points, and it can also boil down what the other person said to their true intent (often with a note of displeasure). It is not, however, used to give an appositive explanation of a single word, so in that use it cannot replace すなわち.
• 3.
  ということだN3ということだ also has a hearsay sense ('it is said that...'). In its rephrasing and summarizing senses it is close to すなわち, but it attaches to the plain form and sits at the end of the sentence while すなわち comes before the rephrased content, so the different placement rules out direct substitution; it is neutral in register, whereas すなわち leans toward written style.
• 4.
  というのはN3というのは opens a sentence to supply the reason for the previous one, typically pairing with 〜からだ or 〜のだ at the end. すなわち only rephrases or summarizes preceding text and cannot introduce a reason, so the two are not interchangeable, and すなわち leans toward written style.
• 5.
  いわゆるN2いわゆる is a prenominal word used only before nouns, signaling that what follows is a name commonly used in society, with a neutral register. すなわち links back to preceding text to rephrase or summarize it, can appear mid-sentence or at the start of a sentence, and leans toward written style — the two cannot be interchanged.

つまり also links two equivalent statements but works in any register, and 要するに sums up the gist, often casually, whereas すなわち marks a strict, formal equivalence. ということだ is a sentence ending that states a conclusion or hearsay, というのは introduces an explanation or reason, and いわゆる means 'so-called' before a familiar term, so these three do not act as a connective of equivalence the way すなわち does.