What's this part of the nose called?

I’m having some confusion about the word nostril. I always thought it was it actual channel or cavity or opening in the nose. But some people seem to refer to the outside part of the nose (left and right) as nostril, the two “flaps” for lack of a better word.

I mean this:

I’ve always called the B part nostrils, but I actually don’t know exactly what I would call the A part.

Another word that’s also associated with this is the nose’s vestibule, but as far as I can tell that also refers to the channel or opening, not to the outer or side part.

Who can shed some light on this? What are the right terms for the A part and the B as separate things?

It's called the nasal ala (adjectival form: alar - thanks @tchrist; plural: alae - thanks Deepak) or wing of the nose.

It refers to the wing-shaped structures that make up the walls of the nostrils. It is the lateral surface of the external nose that is also referred to by the name "Wing of the nose" or "Nasal ala."

- bodterms.weebly.com

The anatomical term is ala of nose:

• The ala of the nose (wing of the nose) is the lateral surface of the external nose, cartilagenous in makeup, and which flares out to form a rounded eminence around the nostril. It is formed from the lateral nasal processes.

(Wikipedia)

^{Illustration from Richard E. Davis, MD; Glossary of Terms Commonly Used in Primary & Revision Rhinoplasty.}