The word homily has a Greek origin. Holilos is the word for “crowd” or “assembly.” The verb form (homilein) means “to address.” Also related is the Greek word homilia, which means “conversation.” So a homily has to do with speaking to an assembly. Today, the word homily is used to refer to a sermon or spiritual lesson. In modern seminaries, a course on homiletics is a course on preaching.
Generally speaking, a homily may be thought of as a sermon, but there are some different uses and nuances of the term. A sermon might be considered a broader term for an address that could include doctrinal teaching. The word homily is usually used to refer to a shorter message of a devotional, inspirational, or moral nature. In Roman Catholic, Orthodox, Anglican, and Lutheran services, the homily is a brief commentary to help the congregation understand the passage that has been read during the service.
The word homily can also be used to denote a moralizing speech delivered by someone outside of a church service, in much the same way that the word sermon or preach can be used in other, non-ecclesiastical settings. A college student might say something like, “I hate going home on break because my dad will preach a sermon to me about my long hair,” or, “My dad offered a homily on the importance of money management before sending me off to college.”
In the final analysis, homily is a rather loose term. It is often used interchangeably with the word sermon. If a Roman Catholic attended a traditional Protestant service, he might refer to the sermon as the homily.
In Protestant services, the preaching of the Word of God is usually the central event, and it is not uncommon for sermons to last at least 30 minutes; some may run for 45 minutes or even close to an hour. In Roman Catholic services, the celebration of the Mass is the central event, and the shorter homily is simply one of many components of the service.