In classical studies the term epyllion (Ancient Greek: ἐπύλλιον, plural: ἐπύλλια, epyllia) refers to a comparatively short narrative poem (or discrete episode within a longer work) that shows formal affinities with epic, but betrays a preoccupation with themes and poetic techniques that are not generally or, at least, primarily characteristic of epic proper.
Ancient Greek ἐπύλλιον (epyllion) is the diminutive of ἔπος (epos) in that word's senses of "verse" or "epic poem"; Liddell and Scott's Greek–English Lexicon thus defines ἐπύλλιον as a "versicle, scrap of poetry" or "short epic poem", citing for the latter definition Athenaeus, Deipnosophistae 2.68 (65a–b):
This is in fact the only ancient instance of the word that shows anything approaching the connotations with which it is most often employed by modern scholars, and epyllion did not enter the common language of criticism until the 19th century. Wolf was apparently responsible for popularizing the term, for two of his essays from early in that century are referred to by titles including epyllion: Ad Scutum Herculis epyllion Hesiodo subditum animadversiones (Observations on the Shield of Heracles, an Epyllion Falsely Attributed to Hesiod) and Theocriti idyllia et epyllia (The Idylls and Epyllia of Theocritus). The locus classicus for the sense of epyllion as a hexametric mythological poem that is not only comparatively short, but also imbued to some extent with the characteristics of Hellenistic poetry is Moritz Haupt's 1855 study of Catullus 64, but it is likely that Haupt was using a term that had in the preceding decades become common to discussions of the shorter narrative poems of the Alexandrians.
In the early 20th century the first studies specifically devoted to the concept of the epyllion were undertaken, with Leumann's work on Hellenistic epyllia, Jackson's study of the possible Roman examples, and Crump's attempt at a diachronic study of the epyllion as a single genre whose history could be traced from the Greek poems of the Hellenistic period through the Augustan period's Latin texts. The exact meaning and applicability of the term epyllion has remained a matter of dispute, and Richard Hunter's recent appraisal summarizes well the current opinion regarding epyllia:
Even if the term "epyllion" has no ancient authority, there has seemed to be a phenomenon which cannot be ignored. Modern discussion has, however, been bedevilled by the grouping together of poems so diverse as to render that grouping almost meaningless, however many points of individual contact they may share.
An epyllion is, in its most basic definition, a narrative poem written in dactylic hexameters that is comparatively short. There is disagreement about whether the term should also be applied to works written in elegiac couplets. The exact meaning of "comparatively short" varies among modern scholars, with some considering Theocritus, Idyll 13 (75 lines) an epyllion, while Eratosthenes' Hermes is commonly classed as an example, even though at some 1,600 lines it would probably have taken up two papyrus rolls. A similar variation in lengths is found in epyllia that form episodes within larger works. Virgil's Nisus and Euryalus digression in the Aeneid totals 73 verses and is sometimes considered an epyllion, while the so-called Aristaeus-epyllion (Georgics 4.315–558) is considerably more substantial and reminiscent of independent epyllia from the Hellenistic period.
Callimachus, Hecale fr. 1 Hollis = 230 Pf.:
Catullus 64.50–54:Philitas, Hermes
Alexander Aetolus, Fisherman
Theocritus 13, 22, 24, 
Eratosthenes, Hermes (debated)
Vergil, Georgics 4.315–558: the Aristaeus-epyllion
Vergil, Aeneid 9.182–234: Nisus and Euryalus
Ovid, Metamorphoses 8.611–724: Baucis and Philemon
Coluthus, Rape of Helen
Musaeus, Hero and Leander