A Theotokion is a hymn (troparion or sticheron) to the Theotokos (Birthgiver of God), which is read or chanted during the Divine Services (Canonical hours and Divine Liturgy) of the Eastern Orthodox and Eastern Catholic churches.
In the fifth century, Nestorius, Patriarch of Constantinople (428 - 431), publicly attacked the use of the term Theotokos to describe the Virgin Mary. His opinion caused a stir in the church and led to his deposition as Patriarch. The reason for this is that the term "Theotokos" is understood by Orthodox Christians to have not only pious but also theological significance: in calling the Virgin Mary the "Birthgiver of God," it is affirmed that Christ possesses a human nature as well as a divine nature (as opposed to being purely divine). This is an essential understanding in the Orthodox doctrine of theosis. Thus the title "Theotokos" is as much a statement about Christ and the incarnation as it is about the Virgin Mary herself; it is this particular view of the incarnation that allows Mary to bear this title.
After the defeat of Nestorianism at the First Council of Ephesus in 431, the use of theotokia during the course of the Divine Services gradually increased. Today, every single service of the Orthodox Church contains theotokia. The inclusion of Theotokia in every service is sometimes accredited to Peter the Fuller, the monophysite Patriarch of Antioch (471 - 488), an ardent opponent of Nestorianism.
Theotokia often occur at the end of a series of troparia or stichera, usually after the verse: "(Glory to the Father, and to the Son, and to the Holy Spirit,) Both now and ever, and unto the ages of ages. Amen."
A Stavrotheotokion is a hymn to the Theotokos that also refers to the Crucifixion of Christ. The correlation between the Theotokos and the Cross is natural because of the Virgin Mary's standing by the Cross throughout the Passion. Stavrotheotokia occurs most commonly on Wednesdays and Fridays, days which are dedicated to the commemoration of the Cross.
The theotokion that occurs at the end of "Lord, I Have Cried" at Vespers is called a Dogmaticon because the text of the hymn deals with the mystery of the Incarnation of Christ. It is during the Dogmaticon that the Little Entrance is made during Vespers. The Dogmaticon is often chanted in a solemn manner, and while the choir is singing it the Deacon or Priest will cense the icon of the Theotokos on the Iconostasis. A Little Entrance is also made during the Divine Liturgy while the choir chants the theotokion that ends the Beatitudes.
The longest and most popular devotion involving Theotokia is the Akathist to the Theotokos. This is solemnly chanted on the Fifth Saturday of Great Lent, and many other times during the year as both public and private devotions.