How many wars have to be going on for it to be declared a 'world war'?

As we know there are many wars going on at present and I wondered how many wars constitute a world war, my partner thought it was when countries form allies against other countries...but that's happening now. After all the UN are a group of coalition/allied forces and they are involved on more than one front. So when does wars become a world war? I hope I framed my question correctly.