Share of wallet (SOW) is a survey method used in performance management that helps managers understand the amount of business a company gets from specific customers.
Another common definition is the following: Share of Wallet is the percentage ("share") of a customer's expenses ("of wallet") for a product that goes to the firm selling the product. Different firms fight over the share they have of a customer's wallet, all trying to get as much as possible. Typically, these different firms don't sell the same but rather ancillary or complementary product.