A diary (also called journal) is a record (originally in handwritten format) with discrete entries arranged by date reporting on what has happened over the course of a day or other period. A personal diary may include a person's experiences, and/or thoughts or feelings, including comment on current events outside the writer's direct experience. Someone who keeps a diary is known as a diarist. Diaries undertaken for institutional purposes play a role in many aspects of human civilization, including government records (e.g., Hansard), business ledgers and military records. In British English, the word may also denote a preprinted journal format.

Today the term is generally employed for personal diaries, normally intended to remain private or to have a limited circulation amongst friends or relatives. The word "journal" may be sometimes used for "diary," but generally a diary has (or intends to have) daily entries, whereas journal-writing can be less frequent.

Although a diary may provide information for a memoir, autobiography or biography, it is generally written not with the intention of being published as it stands, but for the author's own use. In recent years, however, there is internal evidence in some diaries (e.g., those of Ned Rorem, Alan Clark, Tony Benn or Simon Gray) that they are written with eventual publication in mind, with the intention of self-vindication (pre- or posthumous) or simply for profit.

By extension the term diary is also used to mean a printed publication of a written diary; and may also refer to other terms of journal including electronic formats (e.g., blogs).