List of unusual units of measurement
An unusual unit of measurement is a unit of measurement that does not form part of a coherent system of measurement; especially in that its exact quantity may not be well known or that it may be an inconvenient multiple or fraction of base units in such systems. This definition is deliberately not exact since it might seem to encompass units such as the week or the light-year which are quite "usual" in the sense they are often used; if they are used out of context, they may be "unusual", as demonstrated by the Furlong/Firkin/Fortnight (FFF) system of units.
Length
Rack unit
One rack unit (U) is 1.75 inches (44.45 mm) and is used to measure rack-mountable audiovisual and computing equipment. Rack units are typically denoted without a space between the number of units and the 'U'. Thus a 4U server enclosure (case) is seven inches (180 mm) high.
Light-nanosecond
The light-nanosecond was popularized as a unit of distance by Grace Hopper as the distance which a photon could travel in one thousand millionth of a second (roughly 30 cm or one foot): "The speed of light is one foot per nanosecond." In her speaking engagements, she was well known for passing out light-nanoseconds of wire to the audience, and contrasting it with light-microseconds (a coil of wire 1,000 times as long) and light-picoseconds (the size of ground black pepper). Over the course of her life, she found many uses for this visual aid, including demonstrating the waste of sub-optimal programming, illustrating advances in computer speed, and simply giving young scientists and policy makers the ability to conceptualize the magnitude of very large and small numbers.
