Real Dictionary

big business
uncountable noun: Big business is business which involves very large companies and very large sums of money.

big business in American English
the largest business organizations regarded collectively and, usually, in terms of having distinctive political and economic interests
noun: large business, commercial, and financial firms taken collectively, esp. when considered as a group having shared attitudes and goals and exercising control over economic policy, politics, etc

big business in British English
noun: large commercial organizations collectively, esp when considered as exploitative or socially harmful