Diathong- A blending of two vowel sounds that begin with the sound of the first vowel and glides into the other for example- ow as in owl, al as in ale it also refers to the end of a word like poor and chair
A diphthong is a combination of two vowel sounds pronounced in one syllable, where the sound starts at one vowel and glides to another within the same syllable. Examples include the "oi" in "boil" and the "ou" in "cloud".