A mil is a unit of distance, most often used to measure geographic distance, very common in Norway and Sweden, though not used in Denmark, and only occasionally used in Finland by the Swedish-speaking population. Today, it measures by definition 10 kilometres, (≈6.2 (statute) miles) but earlier in history it had different values.
The word is derived from the same source as English mile, a measurement which historically has had many different definitions throughout Europe. In Sweden and Norway, a (statute) mile is often called "engelsk mil", lit. English mil(e).
In Norway and Sweden, the old "land mile" or "long mile" was 36,000 feet: because of the different definitions of foot then in use, in Norway this was 11,295 m and in Sweden 10,688 m. (Had the imperial foot been used, the distance would have worked out to 10,972.8 m.) The distance was equal to an older unit of measurement, the "rast" ("rest", "pause"), representing a suitable distance between rests when walking.  See League (unit)
When the Metric system was introduced in Norway and Sweden in 1889 (the actual law having been passed in 1875), the mil was redefined to be exactly 10 km.
A few years later, the metric system was introduced also in Finland. Even there the traditional measure which is called peninkulma in Finnish and mil in Swedish, was then redefined to be exactly 10 km. In Finland, however, it has been much less in use than in Sweden.
The mil is currently never used on road signs and kilometer is the standard for most formal written distances. It is however very common in colloquial speech involving distances greater than ten kilometers. The mil has however not lost all formal uses. Various tax deductions, for example regarding distance traveled by business purposes, are measured in mil by the Swedish Tax Agency (Skatteverket). It is also used in the most common unit for measuring vehicle fuel consumption -- "litres per mil".