Participant Observation is
"A method of research in anthropology which involves extended immersion in a culture and participation in its day-to-day activities" (Calhoun, 2002).
This type of research methodology is used in circumstances where an individual wants to observe a group to which they do not belong without altering the behavior of the group because of their involvement. Because of this, before observations can be noted as being "natural," the observer must immerse themselves in the culture or group they are observing. The extent to which the observers' participation affects their results has been debated over time.
For more information on participant observation, review the resources below: