Third-party data is data that is collected and compiled on a large, broad scope, that usually holds information on a large group of people rather than a specific customer base. It can be sold and purchased by businesses to help with marketing, advertising analysis and other purposes.
An example of third-party data is data compiled over 5 years about temperature patterns in a specific area. A company that sells home heating and cooling systems may wish to acquire this third-party data to see how its marketing efforts would best be used in that specific area.
Third-party data contrasts with first-party data in that first-party data is collected directly from a customer/user, and directly by a company. For example, in the example above, the email address list of all current clients of the heating and cooling systems company would be first-party data since it was directly collected from actual customers.