In countable|lang=en terms the difference between metonymy and onomatopoeia
is that
metonymy is (countable) a metonym while
onomatopoeia is (countable) a word that sounds like what it represents, such as "gurgle" or "hiss".
As nouns the difference between metonymy and onomatopoeia
is that
metonymy is the use of a single characteristic or name of an object to identify an entire object or related object while
onomatopoeia is (uncountable) the property of a word of sounding like what it represents.