Triphthong

In phonetics and phonology, a triphthong is a monosyllabic sound which functions as a single phoneme and is most of the time regarded as a single sound, while it is in fact a succession of three distinct successive sounds, each of them having its own quality. Most of the time, the first and last sound making up the triphthong are real vowels while the second sound is more or less a semivowel (as in  aua), or there are two semivowels with a real vowel in between (as in iei).

Occurrence
Triphthongs occur in the phoneme inventory of many languages. In English, they can be heard in words such as hour and fire. In Spanish, sound sequences such as iai and iei are very common. However, it is the phonetic context (i.e. whether the vowels are closed or open, whether the first vowel is tonic or not) which determines if these sequences are articulated as triphtongs or as combinations of a single vowel and a diphthong with a hiatus in between. In Punjabi, the tripthong aaiaa is itself a lexeme which means "came". The triphtong oiiaa also occurs here, for example in rassooiiaa which means "cook".