noun which may take the genitive case particle 'no', noun
oral; inside the mouth
Used in medical, dental, or formal contexts to refer to the interior of the mouth. Often appears in compounds like 口内炎 (mouth ulcer) or 口内検査 (oral examination).
口内炎ができて、食事がつらい。
I have a mouth ulcer and eating is painful.
歯科医が口内を詳しく調べた。
The dentist examined the inside of my mouth in detail.
口腔 is a more technical medical term for the oral cavity, while 口内 is slightly more general and can be used in everyday contexts like 口内炎.
Compound of 口 (mouth) and 内 (inside). The reading こうない is the on'yomi of both characters.