This depends on your definition of informal terms like "letter", "character" etc. The typographic term for combinations like this is "digraph". (Wikipedia's de

This depends on your definition of informal terms like "letter", "character" etc. The typographic term for combinations like this is "digraph". (Wikipedia's definition: "A digraph [...] is a pair of characters used in the orthography of a language to write either a single phoneme [...] or a sequence of phonemes that does not correspond to the normal values of the two characters combined".) Whether digraphs have separate keys on a keyboard, are treated as distinct for the purposes of alphabetisation, whether speakers of the language think of them as separate "letters" when spelling out a word and so on, are all separate issues and varies between languages (or, more precisely, between the conventions for writing a certain language).