The lowercase one is the diary, and the uppercase one is the person’s name, because people’s names are usually in capital letters.
Dear Diary means dear Delvey.
DIARY, an English word, noun, means "diary, diary; diary" when used as a noun.
Future Diary future diary
Dear Diary dear diary; ; Primary school student's diary
Site diary site project log; Covered area
Extended information:
1. The basic meaning of diary is "diary, diary book", It refers to a record of what happens every day, especially personal feelings or private issues that you don't want to tell others, or a notebook used to record these things. The extension of diary can refer to "schedule".
2. Diary is a countable noun, and its plural form is diaries, but diary often appears in the singular form.
3. Diary is often used with the verb keep to mean "writing a diary".
4. When expressing "a diary", use an entry of diary.