Alliteration
Repetition of the same consonant in nearby words, creating a sound effect that imitates or underlines meaning.
Alliteration repeats a consonant (or consonant cluster) in nearby words: the sibilants of Racine's famous "pour qui sont ces serpents qui sifflent sur vos têtes ?" imitate the very hissing the line evokes. When sound imitates sense this way, it's called imitative harmony.
A flagship figure of poetry, it also works prose under the surface: discreet alliteration binds a sentence's words and makes it memorable without the reader knowing why. The danger is overdose — too-visible alliteration turns the sentence into a tongue-twister and the effect into a gag. Its vowel twin is assonance.
Example
"Peter Piper picked a peck of pickled peppers."