A seminar is a course offered to between 12 to 18 students, where a primary goal of the course is discussion. Such courses are usually designated with an S, such as ENGLISH 89S, but there are some that do not carry an S designation in the course listing. If you look on the DukeHub listing of courses, all seminars, whether they have an S or not, will have "seminar" listed in the course characteristics.
Note: the fact that a course has a small enrollment does not mean that it is taught as a seminar or will meet the Small Group Learning Experience requirement.
While there is no set format for seminar courses, they do share some features in common that distinguish them from many other courses that you will take at Duke:
Note: While enrollment in seminars is limited, it does not follow that all courses with enrollments below 18 are seminars. When an instructor chooses to offer a course as a seminar, she/he intends for the course to be relatively small but also to involve students in regular class discussion.