Diary basically means "diary, diary", which refers to a book that records what happens every day, especially your personal feelings or privacy issues that you don't want to tell others, or is used to record these things.
Diary is a countable noun, and its plural form is diaries, but diary often appears in singular form.
Diary is often used with the verb keep, which means "keeping a diary".
Discrimination of word meaning:
Diary and journal have the same meaning as diary. The difference is:
Diary usually refers to "not leaking a day"; Journals tend to be "reading notes", not necessarily written every day, and not as private as diaries. Often refers to "an objective record of daily trivia" and "reading essays".
For example:
She? Keep one. Diary? For what? Is it over? Twenty? A few years. She has kept a diary for more than twenty years.
He has kept a diary for three months. He has been writing reading notes for three months.