Oxford English Dictionary 1. The use of symbols in a story, picture, etc., to convey a hidden or ulterior meaning, typically a moral or political one; symbolic representation.

2. A story, picture, etc. which uses symbols to convey a hidden or ulterior meaning, typically a moral or political one; a symbolic representation; an extended or continued metaphor.

A narrativetechnique in which characters representing things or abstract ideas are used to convey a message or teach a lesson. Allegory is typically used to teach moral, ethical, or religious lessons but is sometimes used for satiric or political purposes.
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Examples:

Plato, Allegory of the Cave in The Republic (380BCE)

Dante, Divine Comedy (1320)

Spenser, The Faerie Queene (1590, 1596)

John Bunyan, The Pilgrim’s Progress (1678)

George Orwell, Animal Farm (1945)

William Golding, Lord of the Flies (1954)

C. S. Lewis, The Chronicles of Narnia (1950-56)

Many other examples, but 20th-century literary criticism often disparaged allegory as too artificial and limiting a form. Some texts that readers like to consider allegories, like J. R. R. Tolkien's The Lord of the Rings, was not considered an allegory by its author.