He is writing a thesis on the octovocalic theory.

A diphthong refers to the phenomenon where two vowels are pronounced consecutively within a single syllable.

A back vowel is a sound where the back of the tongue approaches the palate.

Back vowels are vowels that are pronounced using the back of the tongue.

A front vowel is a vowel that is articulated with the front part of the tongue.

The central vowels in Japanese are 'a', 'e', 'o'.

A central vowel is a vowel that is pronounced in the central part of the tongue.

Front vowels are pronounced using the front part of the tongue.

I hate the feeling of a blocked nasal cavity when I catch a cold.

He decided to see a doctor because he has a problem with his nasal cavity.