Earlier this semester, I was in a group which gave a presentation on cohort studies. In this study type, the researcher identifies a group (cohort) with similar characteristics, and observes whether or not similar developments occur across the group over time. If so, it is possible that the characteristics have an influence on the developments.
This type of study has been employed by scientists since the 1700s, when James Lind used it to discover treatments for scurvy. Although cohort studies are used in many disciplines (including library science) the bulk are still performed in the areas of medicine and health science. This makes it a useful term for a medical librarian to be acquainted with (especially when searching for particular types of studies).
For more about cohort studies, try looking at these articles: