One letter refers to two sounds — what is it called?

Solution 1:

If the individual sounds are phonemes, as represented in the question, then the individual letters used to represent them are referred to in linguistics as graphemes. I have heard combinations of graphemes which represent a single phoneme, which is the phenomenon you're asking about, as multigraphs - I don't know if that is common current use.

As you are no doubt aware, there is no one-to-one relationship in English between phonemes and either graphemes or multigraphs. 'Eighth' is a particularly interesting example, where the first four graphemes represent - depending on accent - either a single vowel sound or a diphthong, and the remaining two appear to simultaneously represent two different sounds, as you can't assign the /t/ sound to the <t> grapheme without breaking up the <th> multigraph, so while 'multigraph' covers one part of what you're asking about, it doesn't cover the specific situation you referred to.