A Case Study Method refers to a research procedure in which a single person or small group is studied in detail. The method does not allow conclusions about cause-and-effect relationships, and findings can be generalized only with great caution which is in contrast with Single-Case Experimental Design
Other definition:
Case study method refers to a research method consisting of the intensive description or study of one person,usually a client or patient who is in treatment; an in-depth examination of one person or one group. It is a research procedure in which a single person or small group is studied in detail. The method does not allow conclusions about cause-and-effect relationships, and findings can be generalized only with great caution