tri·graph 
(trīgrăf′)
(trīgrăf′)n.
1. Three letters spelling one consonant, vowel, or diphthong, such as Sch in Schiller or igh in high or thigh.
2. A group of three letters, especially of frequent occurrence in a given language, as the or ing in English or gli in Italian.
3. Any combination of three letters of an alphabet. Also called trigram.
- hydroforming
- hydrogen
- hydrogenase
- hydrogenate
- hydrogen bomb
- hydrogen bond
- hydrogen bromide
- hydrogen chloride
- hydrogen cyanide
- hydrogen fluoride
- hydrogen iodide
- hydrogen ion
- hydrogenolysis
- hydrogenous
- hydrogen peroxide
- hydrogen sulfide
- hydrogeologic
- hydrogeological
- hydrogeologist
- hydrogeology
- hydrographer
- hydrographic
- hydrographically
- hydrography
- hydroid