Assonance refers to repetition of sounds produced by vowels within a sentence or phrase. In this regard assonance can be understood to be a kind of alliteration. What sets it apart from alliterations is that it is the repetition of only vowel sounds. Assonance is the opposite of consonance, which implies repetitive usage of consonant sounds.
“A long song”. (Where the ‘o’ sound is repeated in the last two words of the sentence)