Expert Judgment is a term that refers a specifically to a technique in which judgment is made based upon a specific set of criteria and/or expertise that has been acquired in a specific knowledge area, or product area, a particular discipline, an industry, etc. This knowledge base can be provided by a member of the project team, or multiple members of the project team, or by a team leader or team leaders. However, typically expert judgment requires an expertise that is not present within the project team and, as such, it is common for an external group or person with a specific relevant skill set or knowledge base to be brought in for a consultation, Some examples of resources of expert knowledge can be stakeholders, customers, professional and technical organizations, and other miscellaneous industry groups that may provide these types of services for a small fee, or may provider them free of charge (in some cases, only free if one of the members of the project team is a dues paying member of said organization).
This term is defined in the 3rd and the 4th edition of the PMBOK.