A cash cow is a business, product line, or product that spins off a significant amount of cash flow. The concept was originated by the Boston Consulting Group (BCG), which put the cash cow designation into a grid of four quadrants. In the BCG model, the cash flows from any parts of a business designated as a cash cow should be used to fund those parts of the business designated as belonging in one of the other quadrants.
A cash cow is more likely to be located in a mature, low-growth industry, and has a significant share of the market. The market likely has barriers to entry, so that new entrants cannot jump in and take away market share. Given these factors, a cash cow has little need to invest additional funds in the business, and so can instead spin off cash, perhaps in the form of dividends or stock buybacks.
When a competitor wants to attack a company that has a cash cow, one approach is to go after the cash cow in order to reduce its cash flows, thereby starving the parent entity of cash.