Why is the war fought between 1914 and 1918 called the First World War?

The war fought between 1914 and 1918 is called the First World War because of the following reasons-

1. The War was fought on the land and in the air, on the sea and under it. In the War were mobilised all methods of destruction, such as tanks, submarines, gunfire and bombardments.
2. The War began in Europe, but very soon it spread to continents of Asia and Africa as well.