Why was the war of 1914-1918 called the First World War? Is this name valid?

The war of 1914-1918 was undoubtedly world war, because actually the strongest and most developed countries took part there: the British Empire, the Russian Empire, the Ottoman Empire, France, Germany, etc. In total, about 70 million soldiers were involved, of which 10 million were killed. This war was a big conflict between the warring parties, on the one hand the Entente, on the other the Quadruple Alliance.