A combination of two, three, or four letters that stand for a vowel is known as a vowel digraph, trigraph, or quadrigraph, respectively.
Vowel digraphs are pairs of letters that together represent a single vowel sound. Common examples include:
"ai" in "rain"
"ea" in "head"
"ou" in "cloud"
Trigraphs and quadrigraphs follow the same principle but involve three or four letters representing a single vowel sound. While vowel digraphs are more common, examples of trigraphs and quadrigraphs can also be found in certain words.