What is "apolytikion"

The Apolytikion or Dismissal Hymn is a troparion ( hymn) said or sung at Orthodox Christian worship services. The apolytikion summarizes the feast being celebrated that day. It is chanted at Vespers, Matins and the Divine Liturgy; and it is read at each of the Little Hours. The name derives from the fact that it is chanted for the first time before the dismissal (Greek: apolysis) of Vespers. In the Orthodox Church, the liturgical day begins at sunset, so Vespers is the first service of the day. The term apolyikion is used in Greek tradition. In Slavic tradition the term troparion is specifically used to stand for Apolytikion,　whilst troparion is of more generic usage in Greek tradition.

The apolytikion could be compared in the Western liturgy to the collect or post-communion, inasmuch as it changes for each feast-day of the year and specifically commemorates the subject of the feast.