The most important attribute of a world lan­gua­ge is a large number speakers, both as a native lan­gua­ge and as a foreign lan­gua­ge. World lan­gua­ges are thus the most spoken lan­gua­ges in the world.

Some also see the geographic distribution of a lan­gua­ge as critical when determining if it is indeed a world lan­gua­ge. According to this theory, there are only 5 world lan­gua­ges: English, Spanish, French, Arabic and Portuguese. That is because only these lan­gua­ges are used as official lan­gua­ges in countries on different continents.

The international importance of a lan­gua­ge is determined by how much this lan­gua­ge is used as a lingua franca in inter­natio­nal contacts, business, science and culture.

Since Hindi, Bengali, Urdu and Indonesian are only rarely used inter­natio­nally, they are generally not seen as world lan­gua­ges, despite their large number of speakers.

Wikipedia also has a good article about the world lan­gua­ges.