Cogency is a formal noun. If a statement or an argument has cogency, it seems reasonable and correct — it hangs together, the reasons support the claim, and a careful listener can see why it works. We are talking about the strength of the thinking, not about a loud voice or a famous name. You will almost never hear cogency in a shop or on the bus. It lives in essays, law, and academic talk: "the cogency of her argument," "his points lacked cogency." The everyday way to say it is that an argument is strong, clear, or convincing. The matching adjective is cogent. Though actually, people sometimes praise a speech as if it had cogency when it was only fluent or confident. Strict teachers keep the word for arguments that are logically tight, not merely smooth.

說服力（指論點站得住、合理，不是會說話而已）