The word diphthong (also spelt dipthong) comes from the Greek δίφθογγος meaning with two sounds, or with two tones.
You’ll find that the vowel sound in each of these words changes considerably when you say the word. You’ll also notice that the shape of your mouth changes mid-way through the pronunciation of the vowel.
For example, take the word, ear. If you say it slowly you’ll notice that your mouth starts off wide and then opens and narrows.
And there you have your dipthong! Two vowel sounds in one syllable.
Diphthongs in Detail
To put it into more formal terminology, a diphthong is a monosyllabic vowel combination involving a quick but smooth movement from one vowel to another, often interpreted by listeners as a single vowel sound or phoneme.
The term is sometimes also used to refer to sequences of vowel letters, or diagraphs (conventional two-letter sequences with special values), for example, the ea of bread, pronounced as a single vowel; particularly so when written as a ligature, as æ in Cæsar.
While pure vowels are said to have one target tongue position, diphthongs have two target tongue positions.
Pure vowels or monothongs (also spelt monophthong) are represented in the International Phonetic Alphabet by one symbol. e.g.
sum – /sʌm/
Dipthongs are represented by two symbols with the two vowel symbols representing approximately the beginning and ending tongue positions.