Define the term Elegy. How is Shelley's "Adonais" an elegy ?

Elegy: : a sad poem or song : a poem or song that expresses sorrow for someone who is dead

http://www.merriam-webster.com/dictionary/elegy

The overarching form of the poem is a pastoral elegy, meaning that a shepherd of sorts is mourning the death of another. Literarily speaking, the function of pastoral poetry is reflexive in that it uses older traditions to make complex emotions seem simpler. The Greek legend of Adonis is a tale about a handsome youth who was equally admired by Aphrodite (Urania), Queen of Love, and by Persephone, Queen of Death. (Shelley makes Urania into Adonis’ mother in this elegy.) Unable to agree on which Goddess shall have him, Zeus decided he would spend half the year on Earth with Aphrodite (the spring and summer) and half the year in the underworld with Persephone (autumn and winter). During a summer hunt, Adonis pierced a boar with his spear, wounding but not killing the beast. In retaliation, the boar charged Adonis and stabbed him with his tusk, causing a lesion that would eventually kill the young and beautiful prince. It was said that every year the Greek women would mourn for Adonis when he died, then rejoice when he was resurrected (in the form of the windflower). Using this myth as the central theme in the elegy, Shelley is hoping, or suggesting, that Keats shall be as immortal as the young Adonis. Beyond the obvious parallel that both were taken at a young age, Shelley uses this poem to exhort readers to mourn him in his death, but hold onto him in memory and rejoice in his virtual resurrection by reading his words.