(ひみつ) The common word for secret. A 秘密 is a 秘密 regardless of how many people know it – or even if nobody knows it! The headline, “Scientist attempts to discover the secret to eternal life!” would use 秘密.   $$$
(ないしょ) Usually defined as ‘confidential,’ but that makes it sound more serious than it is – like a spy document or something. Actually, 内緒 is kind of a kids’ word. “Keep this information between us two.”   KIDS