It is considered a World War simply due to the scale. Wars before the Seven Years War were smaller due to the main tactical concepts of the time period, which focused on "scientific" warfare for the sake of limiting bloodshed. However the Seven Years War was larger than any war before it due to the number of players involved. Prussia and Britain were allied and fighting France, Austria, and Russia. It was so large, that the colonial powers were using colonies to fight colonies, known in the US as French & Indian War.

To /u/TheBroMaster16, I wouldn't say that just because X and Y nation weren't involved, it doesn't mean it isn't a true World War. Further, Britain and France were involved in India at this time period. It is meant to express the range of the war rather than the players involved.

Even though it was fought all over the Earth (North America, Europe, India, Indonesia, etc), a true "world war" is defined by the involvement of all the world's powers. The Seven Years' War did not involve the Mughal Empire (now India) and the Qing Dynasty (now China) which I think were both very powerful on the world stage.