diphthong

[P2052 at AOL.COM]

But they're not the same.  In a diphthong, one can hear, more or less, the
gliding from one sound to the other, as in boy ('boi ee); thus, the two
graphemes represent distinct phonemes, which, though blended together, retain
their individuality.   In a digraph, however, separate graphemes represent
only one phoneme,  as with ea in "meat" (one hears only "e") or the ch in
chair, in which case, a completely different phoneme  is heard--that is, one
which is not characteristic of the sounds represented by the individual
graphemes.  The word cluster refers to two or more contiguous consonants, as
with the str in street or the shr in shrimp.  In fact, in most discussions of
clusters, three-consonant ones are always given as examples.

I haven't kept up with the elementary school reading textbooks, but I would
be surprised to find that these very different concepts have all been lumped
together under the label of cluster or digraph.
                        PAT