Q. 1. What is a homily versus a sermon? Which word should be used in the Catholic Church?
A. 1. First of all, it should be said that many people consider the word "homily" to be synonym to the word "sermon."
Prior to Vatican II, the commonly used word was "sermon." These days, the Church uses the word "homily." This may be the reason why some people believe that the word is interchangeable.
It is generally agreed by the people that a homily is a commentary that follows the reading of Scriptures, especially at Holy Mass. The homily is a scripturally-based reflection. "Since Origen's time homily has meant, and still means, a commentary, without formal introduction, division, or conclusion, on some part of Sacred Scripture, the aim being to explain the literal, and evolve the spiritual, meaning of the Sacred Text." [Catholic Encyclopedia]
A sermon takes the form of a lecture or discourse given for the purpose of providing religious instruction or inculcating moral behavior. The sermon is a talk on any aspect of Christian doctrine. It may or may not relate to the readings.