というのは

The expression というのは is used to define, explain, or clarify something. It has two main usages: explaining the meaning of a word or concept, and providing a reason or explanation for something previously stated.

Defining or explaining a term

When used to define or explain what something means, というのは introduces a topic (usually a word, phrase, or concept) and is followed by an explanation. The sentence often ends with ことだ, ものだ, いみ意味だ, or similar expressions.

This usage is very common in explanations, definitions, lectures, and dictionaries. It means "what is called X" or "the thing known as X."

Providing a reason or explanation

というのは can also be used at the beginning of a sentence to explain the reason for something just mentioned. In this case, the sentence typically ends with からだ or のだ. It translates roughly to "the reason is that…" or "that is because…"

Casual variant: って（いうの）は

In casual speech, というのは is often shortened to っていうのは or simply って（は）.