In business, a cash cow is a product or investment that steadily continues to be profitable.
[business]
cash cow in British English
noun
a product, acquisition, etc, that produces a steady flow of cash, esp one with a well-known brand name commanding a high market share
cash cow in American English
Business; Slang
a profitable asset or resource, as a subsidiary company, producing excess funds that are used to finance investment in other areas, support unprofitable ventures, etc.