A pair of letters that operate together to represent a single sound (e.g., clown, chin). In a vowel digraph, a pair of vowel letters operate together to represent a single vowel sound (e.g., ow, ee, oo), and a pair of consonants (e.g., ch, th, sh) to give a consonant digraph.
See Alphabet, Consonants, Grapheme, Vowel-to-consonant ratio, Vowels