What is the difference between onomatopoeia and alliteration?

Buzz is onomatopoeia: a single word whose sound imitates what it names, like clang or splash. Wild winds whisked is alliteration: an initial consonant sound repeated across nearby words. Both are figures of sound, but onomatopoeia works on one word's sound-to-meaning match, while alliteration works on repeated openings across several words. They are not an either/or choice, though, and that is where the confusion starts. Buzzing bees is both at once, the onomatopoeic buzz alliterating with bees, so the useful question is never "which one is this?" but two separate questions asked of the same line.

How do you tell onomatopoeia and alliteration apart in a real line?

Ask the line two questions, one for each figure.

First: does a single word in this line sound like the thing it names? If it does, that word is onomatopoeia (a word whose sound imitates what it names; a trope). Buzz, clang, and splash all pass, because saying the word produces something close to the noise it points at.

Second: do nearby words start with the same sound? If they do, that is alliteration (the repetition of an initial consonant sound across nearby words; a scheme). In "The furrow followed free" (Coleridge, The Rime of the Ancient Mariner, 1798), furrow, followed, and free all open on the f-sound. No single word imitates a noise here, so this line is alliteration without onomatopoeia.

The trap in the second test is letters. Alliteration tracks the opening sound, not the opening letter. Phone and fiddle alliterate, because both begin with the f-sound even though phone is spelled with a p. City and cat do not alliterate, even though both start with c, because city opens on an s-sound and cat on a k-sound. Read the words aloud and listen for the sound; the spelling will mislead you.

	Onomatopoeia	Alliteration
What repeats or imitates	A word's sound matches its meaning	An initial consonant sound recurs
Unit it works on	A single word	Across two or more nearby words
Test question	Does this word sound like what it names?	Do nearby words start with the same sound?
Sourced example	tinkle, tintinnabulation (Poe, "The Bells," 1849)	"The furrow followed free" (Coleridge, The Rime of the Ancient Mariner, 1798)

Can a word or phrase be both onomatopoeia and alliteration at once?

Yes, and this is the case the two-question test is built for. The figures stack rather than compete. Buzzing bees carries the onomatopoeic buzz and the b-alliteration running across buzzing and bees. One short phrase, both figures, no contradiction.

Tennyson does the same thing in "The murmuring of innumerable bees" (The Princess, 1847). Murmuring imitates the low, continuous sound bees make, so it is onomatopoeia. It also alliterates: the m-sound carries from murmuring into innumerable. The single word answers yes to both test questions at once.

They stack because they measure different things. Onomatopoeia is a fact about one word, whether its sound matches its sense. Alliteration is a fact about a group of words, whether they share an opening sound. An onomatopoeic word that happens to begin like its neighbors satisfies both conditions at the same time, with no tension between them. Poets reach for this deliberately, pairing an imitative word with neighbors that share its opening sound so that a single line of onomatopoeia and alliteration combined does double duty.

Why is onomatopoeia a trope and alliteration a scheme?

The surface difference (one word versus several) sits on top of a deeper one. A scheme is a figure of arrangement, where the sounds or words sit. A trope is a figure of meaning, a word standing in a special relation to its sense.

Onomatopoeia is a trope. Its word's sound carries part of its meaning. Splash does not just point at a splash the way water-landing would; the word's noise is doing some of the naming. The sense and the sound are tied together, and that link is what makes it a figure of meaning.

Alliteration is a scheme. The words in "The furrow followed free" mean exactly what they mean; furrow would name the same thing if the surrounding words started with any other letter. The figure lives entirely in the placement of repeated initial sounds, not in any shift of meaning. That is why onomatopoeia works on a single word, where sound meets sense, while alliteration works across several, where the arrangement of openings is the whole effect.

What about assonance, isn't that a sound figure too?

It is the third figure writers mix in with these two. Assonance is the repetition of vowel sounds within nearby words, as in "the rain in Spain," where the long a recurs across rain and Spain. It is a scheme of sound, like alliteration, but it tracks vowels instead of opening consonants.

That gives you a one-line test for all three. Onomatopoeia: does a single word sound like what it names? Alliteration: do nearby words share an opening consonant sound? Assonance: do nearby words share an interior vowel sound? Onomatopoeia is about one word, the other two about sounds recurring across words, split by where the matching sound sits. The cleanest separation runs between a single word imitating a sound and a repeated vowel running through a phrase, which is the pair most often confused once alliteration is set aside. The point that carries across all three is the one to keep: these are not rival labels you sort a line into, but different things a line's sound can be doing at the same time, so the question is never "which one is this?" but "what is the sound doing here, and is more than one figure at work?"