Outsourcing is an agreement in which one company hires another company to be responsible for a planned or existing activity which otherwise is or could be carried out internally, i.e. in-house, and sometimes involves transferring employees and assets from one firm to another. The term outsourcing, which came from the phrase outside resourcing, originated no later than 1981. The concept, which The Economist says has "made its presence felt since the time of the Second World War", often involves the contracting of a business process (e.g., payroll processing, claims processing), operational, and/or non-core functions, such as manufacturing, facility management, call center/call center support.

The practice of handing over control of public services to private enterprises (privatization), even if conducted on a limited, short-term basis, may also be described as outsourcing.

Outsourcing includes both foreign and domestic contracting, and sometimes includes offshoring (relocating a business function to a distant country) or nearshoring (transferring a business process to a nearby country). Offshoring and outsourcing are not mutually inclusive; one can exist without the other. They can be intertwined (offshore outsourcing), and can be individually or jointly, partially or completely reversed, in methods including those known as reshoring, inshoring, and insourcing.