noun. Three letters spelling one consonant, vowel, or diphthong, such as Sch in Schiller or igh in high or thigh.

noun. A group of three letters, especially of frequent occurrence in a given language, as the or ing in English or gli in Italian.

noun. Any combination of three letters of an alphabet.

noun. A combination of three letters to represent one sound; a triphthong, as eau in beau.

noun. Three letters united in pronunciation so as to have but one sound, or to form but one syllable, as -ieu in adieu; a triphthong.

noun. A specific sequence of three letters, especially one used collectively to represent a single phoneme.

"The difference may be in the language to describe the concepts (rods), or the subjective value of even studying them (trigraph, linguals)."

ligature  triphthong  

triphthong