Throughout the history of human beings, there have been countless wars among on, two or multiple countries, but only two of these have been dubbed "world" wars. Which leads me to my question, is there a specific set of demands that a war has to meet in order to be called a world war or is it decided conventionally?
A war involving many large nations in all different parts of the world. The name is commonly given to the wars of 1914–18 and 1939–45, although only the second of these was truly global.
Is the definition according to the dictionary, however there are no specific criteria involved. The reason that WW1 and WW2 are considered "World Wars" is because the main countries involved in the wars e.g. Britain, France, Germany also pitted their empires into the war.
The British Empire alone in 1920 (shortly after the end of WW1) covered roughly 1/4 of the Earths surface and population and during both WW1 and WW2 fought in theaters across multiple continents, thus making it a truly global war.
World War I was originally called Great War, only after Second World War the denomination was changed. Hence is quite probable than in the future the names will still be subject of change.
Some authors talk about European Civil War instead of World War to cover the period from 1870 to 1945 or 1914 to 1945, including here Russian and Spanish civil wars. Others say that this period was actually a religious war between communism and nationalism.
Summary: Names of events and periods are constantly changing according to new theories, sources of information or events.
They are decided conventionally. For example Americans called "European War" instead of WW1 at the time. There is no specific criterion for it. However, for a war to be world war it should include most powerful and populous countries, multiple continents. And they generally include multiple conflicts between those countries.