An elegy is a kind of poem. Its topic is sad, having a mournful tone, often referring to death. A great deal of thought is put into an elegy, written in memory of a specific person or group of people who have died.
The term "elegy" dates back to the 1510s; it originated in the Middle French, used before that in the Greek and Latin, meaning, literally, "poem or song of lament" (deep mourning often reflected in verse or a dirge). The Greeks often placed elegies on tombs.
Examples of elegies are found in poems such as "Elegy Written in a Country Churchyard" by Thomas Gray; or "O Captain! My Captain" written by Walt Whitman at the death of Abraham Lincoln; "On My First Sonne" written by Ben Jonson at the death of his son Benjamin; and, W.H. Auden's "In Memory of W. B. Yeats."
Your one-line definition is:
An elegy is a sad, mournful poem dealing with death, often written in memory of someone, first offering the lament, praise, and finally, consolation; it has no specific structure as in rhythm (metrical form) or repetition, but is free-flowing.
'elegy'- A mournful or plaintive poem or song, esp a lament for the dead.