A mile is a unit of length or distance that has different values according to the system used.
The word mile comes from the Latin mille meaning one thousand. The original mile was 1000 paces (mille passus) as covered by marching Roman infantry, where 1 pace is equavelent to 2 steps. The Roman mile was 5000 Roman feet (approximately 1618 British Imperial/US yards or 1480 metres).
In the reign of Queen Elizabeth I of England, the mile was decreed by a Statute of Parliament to be 5280 feet long or 320 rods and is therefore known as a statute mile. This measure is still used mainly in the USA and UK. There has been widespread resistance in America and Britain to use the metric system, which continues to this day.
The international statute mile was defined in 1960 as being 1760 yards (or 1,609.344 metres) long with one inch (1/36 yard) being set at exactly 25.4 millimetres. This means that 1 metre is 39.3700787 inches.
The US survey mile is defined in reverse, with 1 metre being equal to 39.37 inches exactly. The US survey mile is therefore 3 mm longer than the international mile at 1,609.347 metres). This unit of measurement is used by US Public Land Survey System.
1759 yards are one yard short of one mile.
The nautical mile is a geodetic measurement originally defined as 1 minute of arc measured along a meridian or great circle, or 1 minute of latitude. However, as we now know that the earth is not a perfect sphere the distance varies between 6067 and 6087 feet using the WGS84 ellipsoid model, with an average of 6076 feet.
In Britain the Admiralty nautical mile was defined as 6,080 feet (1,853.184 m) which is approximately one minute of latitude in the southern latitudes of the UK. In the United States, the nautical mile was defined in the nineteenth century as 6,080.2 feet (1,853.249 m). Some other countries had their own definitions of the nautical mile, but it is now internationally agreed to be exactly 1,852 metres or approximately 6,076.12 feet. The international standard was adopted by the US in 1954 and by the UK in 1970.
The nautical mile is used mainly for maritime and aviation purposes because of its relation to the degrees of latitude. A knot is used to describe speed in nautical miles per hour. The term knot is sometimes erroneously used instead of nautical mile (knot being thought a diminutive of nautical). However, the name derives from the ancient practice of throwing a log overboard with a knotted rope attached. The knots were made at fixed intervals and as the rope was paid out the number of knots that passed through a sailor's hands in a fixed time were counted to give a measure of the speed. 
As ships and aircraft travel through a fluid medium their speed is relative to the speed of the water or air and may be referred to as "boatspeed" or "airspeed". Consequently there are several additional definitions of speed which are given in knots: speed of a vessel over the ground (SOG) describes the speed with reference to geographical location and velocity made good (VMG) describes its rate of progress toward a distant point.
A metric mile is often used in international sports (athletics, swimming, speedskating) to denote a 1500 metre event.
Other variations on the mile have been used by the Danish, Dutch, Finnish German, Irish, Italian, Norwegian Polish, Scottish and Swedish and could be from about 1 kilometre up to 12 kilometres in length.