Simple Definition:

Alliteration is a literary device [a technique used in writing to create a particular effect] that involves the repetition of the initial sound of words (consonants or vowels) in a phrase or sentence, usually for emphasis or to create a poetic effect.

Example: Peter Piper picked a peck of pickled peppers. [Repetition of the “p” sound in the words “Peter,” “Piper,” “picked,” “peck,” and “pickled” is an example of alliteration.]

Very Simple Definition:

Alliteration is when words in a sentence start with the same sound.

Example: Sassy Sally sells seashells by the seashore.