This article contains overly lengthy quotations for an encyclopedic entry. (December 2019)
A homily is a commentary that follows a reading of scripture. It is more importantly moral and hortatory. In Catholic, Anglican, Lutheran, and Eastern Orthodox Churches, a homily is usually given during Mass (Divine Liturgy or Holy Qurbana for Orthodox and Eastern Catholic Churches, and Divine Service for the Lutheran Church) at the end of the Liturgy of the Word. Many people consider it synonymous with a sermon.
The English word homily is derived from the Ancient Greek word homilia, which means intercourse or interaction with other people (derived from the word homilos, meaning "a gathering"). The word is used in 1 Corinthians 15:33 ("wicked homiliai corrupt good morals"). The related verb is used in Luke 24:14 (as homiloun), and in Acts 24:26 (as homilei), both used in the sense of "speaking with". The word later came to have a more technical sense. According to The Catholic Encyclopedia, Origen was the first to distinguish between logos (sermo) and homilia (tractatus).
The General Instruction of the Roman Missal (GIRM)
29. When the Sacred Scriptures are read in the Church, God himself speaks to his people, and Christ, present in his own word, proclaims the Gospel. Therefore, all must listen with reverence to the readings from God's word, for they make up an element of greatest importance in the Liturgy. Although in the readings from Sacred Scripture God's word is addressed to all people of every era and is understandable to them, nevertheless, a fuller understanding and effective use of the word is fostered by a living commentary on the word, that is, the Homily, as part of the liturgical action.
65. The Homily is part of the Liturgy and is strongly recommended, for it is necessary for the nurturing of the Christian life. It should be an exposition of some aspect of the readings from Sacred Scripture or of another text from the Ordinary or from the Proper of the Mass of the day and should take into account both the mystery being celebrated and the particular needs of the listeners.
66. The Homily should ordinarily be given by the priest celebrant himself. He may entrust it to a concelebrating priest or occasionally, according to circumstances, to the deacon, but never to a lay person. In particular cases and for a just cause, the Homily may even be given by a Bishop or a priest who is present at the celebration but cannot celebrate. There is to be a Homily on Sundays and holy days of obligation at all Masses that are celebrated with the participation of a congregation; it may not be omitted without a serious reason. It is recommended on other days, especially on the weekdays of Advent, Lent and the Easter Season, as well as on other festive days and occasions when the people come to church in greater numbers. After the Homily a brief period of silence is appropriately observed.
In colloquial, non-religious, usage, homily often means a sermon concerning a practical matter, a moralizing lecture or admonition, or an inspirational saying or platitude, but sermon is the more appropriate word in these cases.