Thick description is described by Lincoln and Guba (1985) as a way of achieving a type of external validity.  By describing a phenomenon in sufficient detail one can begin to evaluate the extent to which the conclusions drawn are transferable to other times, settings, situations, and people.

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The term thick descriptions was first used by Ryle (1949) and later by Geertz (1973) who applied it in ethnography. 

Thick descripton refers to the detailed account of field experiences in which the researcher makes explicit the patterns of cultural and social relationships and puts them in context (Holloway, 1997).

This can be contrasted with thin description, which is a superficial account.  

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References

Geertz, C. (1973). The Interpretation of Cultures. New York: Basic Books.

Holloway, I. (1997). Basic Concepts for Qualitative Research. London: Blackwell Science.

Lincoln, YS. & Guba, EG. (1985). Naturalistic Inquiry. Newbury Park, CA: Sage Publications.

Ryle, G. (1949). The Concept of Mind. London: Hutchinson.

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See also observation