Troparion

Troparion (also tropar; plural troparia) is a type of hymn in Byzantine music, in the Orthodox Church and other Eastern Christian churches. It is a short hymn of one stanza, or one of a series of stanzas; this may carry the further connotation of a hymn interpolated between psalm verses.

The term most often refers to the apolytikion (or "dismissal hymn"), the thematic hymn which closes Vespers. (In Greek churches, the apolytikion troparion is known simply as the apolytikion; in most other churches, it is known simply as the troparion.) This troparion serves as a thematic hymn and is repeated at every service of the day.

Troparia are also found as the stanzas of canons. Such troparia are modeled on the irmoi of the ode.

Troparia are also sometimes used as refrains for chanted psalm verses, though stichera more often serve this function.

Theotokion
A theotokion (or bohorodichnyj) is a troparion to the Theotokos; these hymns are collectively called theotokia.

History
A famous example, whose existence is attested as early as the 4th century, is the Vespers hymn, Phos Hilaron, usually translated as "Gladsome Light" in English; another, O Monogenes Yios, "Only Begotten Son," ascribed to Justinian I (527-565), figures in the introductory portion of the Divine Liturgy. Perhaps the earliest set of troparia of known authorship are those of the monk Auxentios (first half of the 5th century), attested in his biography but not preserved in any later Byzantine order of service.