What is the definition of alliteration?

Alliteration is the repetition of the leading consonant sound in each Word throughout a sentence or a phrase. Alliteration is commonly used in poetry and tongue twisters. It is also sometimes used in advertising taglines and business names to make them more memorable.

Peter Piper picked a peck of pickled peppers.

She sells seashells by the seashore.

Guinness is good for you!

Tom's Timely Tailoring

Happy hunting!

Bye, baby bunting,

Daddy's gone a-hunting,

Gone to get a rabbit skin

To wrap baby bunting in.

Betty Potter bought some butter,

But she found the butter bitter,

So she bought some better butter

To make the bitter butter better.

"Nellie breathlessly beheld a brave bluebird busily bathing in the brimful birdbath. The balmy breeze blew briefly, and the beautiful bluebird bounded onto a big birch tree branch. Then her beloved birds began to beg boldly with bountiful birdcalls, and Nellie became brokenhearted because she beheld that her bird feeders barely had any birdseed."

This is an excerpt from Thank You for the Thistle by Dorie Thurston.
Alliteration is a device used in writing. Alliteration occurs when Words start with similar consonant sounds eg

She sells sea-shells down by the sea-shore. - here the Words start with -s

The wild winds whooshed to the west. - here the Words start with -w