A trigraph (en français, trigramme ; em português, trígrafo) is three characters written to make one sound (phoneme). The Latin-based orthographies for some languages include trigraphs, and in some cases they may be treated as single characters in collation. See also digraph.
Wikipedia, "Trígrafo," http://pt.wikipedia.org/wiki/Trígrafo
______, "Trigramme," http://fr.wikipedia.org/wiki/Trigramme
______, "Trigraph (orthography)," http://en.wikipedia.org/wiki/Trigraph_%28orthography%29