The project management term influencer refers to
the particular person, group, or individual who, while not directly related to the project in question, not a member of the specific project team or a project team leader, and not a financial representative of the company that may be financially responsible for the project, nevertheless for one reason or another may bear significant influence or weight on the project in general, or to the acquisition and purchase of a number of activity related products and services in general. The cause of this influence is typically directly related to the stance or position of this particular person in the pecking order and or chain or command that this person is a part of. This person may hold a particular noteworthy position within the customer organization, and as such, his opinions may in fact hold additional weight. His or her comments and suggestions may both help the progress of a particular project, and in other cases may in fact hinder it in the event his influence causes additional work.
This term is defined in the 3rd edition of the PMBOK but not in the 4th.