Final answer:
An allusion is an indirect reference to another work, often used in English literature. In Shakespeare's Hamlet, the reference to Old Hamlet being poisoned in the castle garden is a biblical allusion to the Garden of Eden.
Step-by-step explanation:
An allusion is an indirect reference, usually to another work, outside of the text, without explicitly naming the reference point. It is a literary device commonly used in English literature. For example, in Shakespeare's Hamlet, the scene of Old Hamlet being poisoned in the castle garden by his brother and being referred to as a "serpent" would be a familiar allusion to the Biblical Garden of Eden for highly religious Elizabethan readers.