The area of the body located underneath where the arm connects to the torso, often appearing when one or both arms are raised, especially when wearing clothes that are sleeveless, strapless, have detached sleeves, have armpit cutouts, or if the character is topless or outright nude.
Hakurei Reimu of the Touhou series is known for a meme involving her armpits since her dress does a lot to emphasize them.
