Stock photography is a collection of photos available for use with projects the original photographers did not intend or know about. They are typically generic images depicting landscapes, cultural icons and people doing common activities and expressing common emotions.
Stock photography is commonly used in advertising, though its use in finished products is controversial. It doesn't reflect the product or service being advertised as well as a custom photo shoot and is preferred for mock-up advertisements.
The name may date back to H. Armstrong Roberts, the founder of one of the first stock photography agencies. It may be a reference to the stock market because the photos are repeatedly traded and sold.