Short answers, pulled from the story.
The metre was redefined by the speed of light on the 26th of November 1983. This change occurred when the General Conference on Weights Measures updated the definition to ensure future technological advancements could not alter the standard.
Before 1983 the metre was defined as one ten-millionth of the distance from the equator to the North Pole. This definition led to the creation of a prototype metre bar made of platinum-iridium kept in France.
Albert Einstein's theory of special relativity published on the 30th of September 1905 showed that length is not an absolute property of an object. The theory established that a ruler appears shorter to an observer moving at high speed relative to the ruler through a phenomenon known as length contraction.
In spherical geometry the shortest distance between two points is a segment of a great circle. A great circle is the largest circle that can be drawn on a sphere and this principle is essential for navigation and astronomy.
In computer science the length of a string is the count of characters it contains. The length of a sequential data structure like an array or list is the number of elements it contains.