There's a number of different definitions for diphthong. One is multiple letters that make one sound, which is how I used it. These are also called digraphs. One is as you state, when two vowel sounds are combined to make one. I don't think that that has another word. Another is the character ``æ'' (``ae'' if that didn't make it -- I wish we could use HTML character entity references here) or ``œ'' (``oe''). Occasionally, you'll see someone refer to a ligature, a physical combination of two characters (like the combinations of ``fi'', ``fl'', and ``ffl'' you sometimes see in typesetting), as a diphthong, but that is incorrect.

To speak more about Richard's diphthongs, those of you who don't know what we mean, say ``ah'', as if you were opening your mouth for the doctor. Now say ``ee'' as in the vowel sound in the middle of ``squeal''. Now say the first quickly followed by the second. Then press them closer and closer together. You'll see that you eventually make the sound ``eye''. That's a diphtong, by Richard's, more correct, definition. Several English vowels are actually diphthongs, like the vowels in ``out'' and ``boy''