Dear diary, you are a written record of things that happen each day. Some people keep a diary while they’re traveling, others keep their secrets in a dairy. Either way, keep out!
Some diaries have a pretty cover and a tiny lock with a key, for keeping the contents safe from prying eyes. You can also call such a book a "journal," although traditionally a diary is written in each day, while a journal might have less frequent entries. The word diary comes from the Latin word diarium, "daily allowance," with its root of dies, or "day."