A nostril is one of the two small openings at the end of your nose. Air goes in and out through them when you breathe, and smells come in that way too. You have a left nostril and a right nostril; together they are your nostrils. We use the same word for people and for animals. It is a plain body word — not fancy, not rude on its own — the word a doctor or a child would both use. Though actually the inside of the word is an old picture: nose plus thirl, an old English piece meaning 'hole', so it is literally a nose-hole. We do not say thirl for 'hole' in ordinary English any more.

鼻孔