Final answer:
The literary device used in the phrase 'United we stand; divided we fall' is Antithesis, which juxtaposes contrasting ideas in a balanced structure.
Step-by-step explanation:
The phrase 'United we stand; divided we fall' uses the literary device known as Antithesis. Antithesis is a figure of speech that juxtaposes two contrasting or opposing ideas, typically within a balanced or parallel phrase or structure. In this case, the idea of being united and consequently standing strong is contrasted with the idea of being divided, which leads to failure. The phrase does not fit the definitions of a pun, idiom, or onomatopoeia. A pun is a play on words that exploits the different possible meanings of a word or words that sound alike but have different meanings. An idiom is a commonly used expression whose meaning does not relate to the literal meaning of its words. Onomatopoeia refers to a word that phonetically mimics or resembles the sound of the thing it describes.