という

という is a versatile particle/quotative that attaches to nouns or clauses to indicate what is being said, named, or believed. It can report hearsay, mark that something is called or referred to as something, or introduce a topic indirectly. It is common in both formal and casual speech, often used to convey information attributed to others, or to define or name something in a concise way. When used after a clause, it marks reported speech or a proposition. After a noun, it can mean 'called ~' or 'the thing called ~'. In combination with こと, it turns into reported content or explanation.