a personal journal (as a physical object)
"I kept my childhood memories safe in a leather-bound diary tucked behind the bookshelf."
A daily log of experiences, especially those of the writer.
"She sat down at her desk every evening to write in her diary about the day's events and her thoughts on them."
In plain English: A diary is a personal notebook where you write down your thoughts and daily events.
"She kept her daily thoughts and memories in a small leather diary."
Usage: Use "diary" to refer to a personal journal where someone records their daily thoughts and experiences privately. Do not confuse it with "daily," which functions as an adjective describing something that happens every day.
To keep a diary or journal.
"She spends her quiet evenings by the window, carefully writing down her thoughts in her new leather-bound diary."
In plain English: To diary something means to write it down in a personal journal.
"She likes to diary her daily thoughts before going to bed."
Usage: Do not use the verb diary to mean keeping a personal record; instead, use the noun form as a gerund, such as "I enjoy diary-ing my thoughts." The correct verb for this action is simply to diary, though it is often replaced by phrases like "keep a diary" or "journal."
Lasting for one day.
"The diary entry noted that our summer vacation was lasting for one day before rain forced us to stay indoors."
"She kept her personal diary in a small leather notebook."
Usage: Use "diary" only as a noun to refer to a personal journal; do not use it as an adjective to mean lasting for one day, which is the incorrect usage of this specific definition. The correct term for something that lasts a single day is "daily."
The word diary comes from the Latin term diarium, which originally meant a daily allowance given to soldiers. It entered English via Late Latin where it began to refer specifically to a record of daily events.