What term refers to a sermon on a moral or religious topic?

The term that refers specifically to a sermon on a moral or religious topic is "homily." A homily typically is delivered during a religious service and aims to provide interpretation or insight into a particular Scripture passage, often applying its lessons to daily life. This reflects its nature as a motivational discussion that encourages ethical behavior and spiritual reflection among the congregation.

The other options do not accurately capture this specific context. A lecture is generally an educational speech that imparts knowledge on a subject but lacks the spiritual or moral emphasis of a homily. A dissertation is a lengthy, formal document or treatise, usually submitted for an advanced degree, focusing on original research and analysis—rather distinct from the didactic and pastoral nature of a homily. An exposition typically refers to a comprehensive explanation or interpretation of a text or concept, and while it may share some similarities with a homily, it lacks the distinct religious framework that characterizes a homily's delivery and intent. Therefore, "homily" is the precise term denoting this type of sermon.