A logbook was originally a book for recording readings from the chip log, and is used to determine the distance a ship traveled within a certain amount of time. The readings of the log have been recorded in equal times to give the distance traveled with respect to a given start position.
Today's ship's log has grown to contain many other types of information, and is a record of operational data relating to a ship or submarine, such as weather conditions, times of routine events and significant incidents, crew complement or what ports were docked at and when. It is essential to traditional navigation, and must be filled in at least daily.
Most National shipping authorities and Admiralties specify that logbooks are kept to provide a record of events, and to help crews navigate should radio, radar or the GPS fail. Examination of the detail in a ship's log is often an important part of the investigative process for official maritime inquiries, in much the same way as a "black box" is used on airplanes (see Mary Celeste). Logbook entries are sometimes of great importance in legal cases involving maritime commercial disputes.
The term logbook has spread to a wide variety of other endeavors, and logbooks are widely used for e.g. complex machines like nuclear plants or particle accelerators where one is more and more using a computer based electronic logbook. In military terms, a logbook is a series of official and legally binding documents. Each document (usually arranged by date) is marked with the time of an event or action of significance.
Commercial ships and Naval vessels often keep a "rough log," - or "scrap log," - a preliminary draft of the ship's course, speed, location, and other data, which is then transcribed as the "smooth log," - or "official log" - the final version of the ship's record. Changes may be made to the rough log but the smooth log is considered permanent and no erasures are permitted. Alterations or corrections in an official logbook must be initialled by the authorised keeper of the logbook and the original data entries which have been cancelled or corrected must remain legible.
Use in fiction 
- The Hornblower series mentions logs to explain plot development, or to make the story more realistic.
- Reading a log can make a dramatic explanation of a mysterious disaster in most sci-fi.
- In Star Trek the Captain's log, a form of ship's log, is used to fill in the audience as to the events in progress, and acts as a more realistic form of soliloquy.
Alternative connotation 
For Amateur radio the logbook is where the hams register their QSO and radio activity. There are several programs to help radio operators in the management of their logbook.
Pilots of aircraft must maintain a logbook (as per FAR part 61.51. in the USA, JAR-OPS 1.970 for Nations enrolled in the JAA) Pilots must log flight time used to meet the requirements for a certificate or rating, and for currency purposes.
In scuba diving, the logbook documents the experience of a diver by logging a diver's dives.
See also 
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* NOAA fisheries logbook guide (broken)