definition
strict definition
seminary
A school training students for religious clergy, usually at higher-education level and usually serving adults.
A seminary is a school which trains students to become members of a religious clergy. It is usually, but not necessarily, at the level of higher education (post-intermediate), and the students are in most cases of adult age.
Preparation for membership in a religious clergy is the defining purpose. The scope is not restricted to one religion or denomination.
Adult enrollment and higher-education level are usual rather than mandatory. General religious instruction without clergy training does not by itself establish a seminary.