In anthropology and other fields, a thick description of a human behavior is one that explains not just the behavior, but its context as well, such that the behavior becomes meaningful to an outsider.
The term was used by the anthropologist Clifford Geertz in his The Interpretation of Cultures (1973) to describe his own method of doing ethnography (Geertz 1973:5-6, 9-10). Since then, the term and the methodology it represents has gained currency in the social sciences and beyond. Today, "thick description" is used in a variety of fields, including the type of literary criticism known as New Historicism.
In his essay "Thick Description: Toward an Interpretive Theory of Culture" (1973), Geertz explains that he adopted the term from philosopher Gilbert Ryle, specifically his lecture "What is le Penseur doing?"