Diary (Lat. diarium, 'a daily allowance for soldiers,' 'diary,' from dies, 'a day') means simply a daily record of events or observations made by an individual. In it the man of letters inscribes the daily results of his reading or his meditations; to the mercantile man it serves the purpose of an order or memorandum book; while the physician finds it indispensable as a register of engagements. Diaries in many forms and sizes are issued every year, containing also so much miscellaneous information that in one book we have at once a diary and an almanac. The Ephemeris of the ancients was originally a military record or journal, a day-book or account-book, also a collection of tables showing the position of the heavenly bodies, but passed into literature to mean a collection of records of what has happened on the same day in various years, or a mere general name for any form of periodical books or magazines.
Diaries have often furnished the historian with invaluable material, supplying the absence of public records, and furnishing minute and intimate details of manners and of motives that do far more to help us to understand the past than more formal records. Such documents as Robert Baillie's Journals, the Diaries of Pepys and Evelyn, and the Journals of Greville are among the most valuable sources of real history. Bacon says 'in sea-voyages, where there is nothing to be seen but sky and sea, men make diaries; but in land-travel, wherein so much is to be observed, they omit it,' but unhappily this no longer holds of modern travellers.