tri·graph \ˈtrīˌgraf, -aa(ə)f, -aif, -ȧf\noun Etymology:tri- + -graph 1.: three letters spelling a single consonant, vowel, or diphthong < sch of schism, cch of bacchic, pph of sapphic, tth of Matthew, and eau of beau are trigraphs > 2.: a cluster of three successive letters < THE is a high frequency trigraph >