Revenue is value of sales generated by a company.
In British English, which is the default flavor of English used in Commonwealth countries like India, turnover means the same thing.
However, over a period of time, the term started being used in other contexts viz. manpower turnover (i.e. how many employees joined versus left a company in a given year), inventory turnover (i.e. how many times did a company sell its inventory in a given year). So turnover has become a bit ambiguous.
Therefore, Revenue has become a more definitive term to use in order to express a company’s sales, which in turn determines its size according to FORTUNE 500, FORTUNE GLOBAL 500 and other popular lists of largest corporations in USA and World.