The term "log" is a nautical one and, oddly enough, was first used aboard marine vessels. Originally, “log” referred to a device used to measure a seagoing vessel’s speed. A board or plank of a specific size and density cut from a tree and attached to a line which had knots tied at regular, measured intervals was tossed overboard and the number of knots passing through a sailor's hands in a specific amount of time indicated the ship's speed — in Knots.
Keeping track of speed, direction, tides and astronomical observations was mission critical for navigation. Over time, the term "log" was applied more broadly to indicate records of events which took place aboard ships at sea as well as in port. Log keeping was later adapted to accurate data collection in other fields; such as land navigation, scientific experiments, and guest registers in public houses.