You're possibly confusing a few different concepts:
A phoneme is an 'underlying sound building block'.
A digraph is a two-letter combination that represents a particular phoneme or has a particular value/use. (You can then have trigraphs etc, but in reality, in common parlance, you can just refer to "letter combinations".) So in your example, you could say that the 'ai' digraph represents the phoneme /aI/.
A grapheme is basically a "distinct letter" although arguably the term doesn't work terribly well in practice (are "é" and "e" separate graphemes or not in English?)
You can then talk about whether a language with an alphabetic writing system has a close grapheme-phoneme correspondence.
In principle, you could talk about a bijective or non-bijective orthography, but in reality these terms are not very useful: practically no language actually has a perfect grapheme-phoneme correspondence and virtually all languages will lie somewhere on a gradient of non-perfect grapheme-phoneme correspondence.
In other words, you're asking for a special term for what is basically the usual case for languages with alphabetic writing systems, so to a large extent, the term is simply "alphabetic writing system".