stop sign
A red road sign. It's usually in the shape of an octagon in most countries, but can sometimes be in other shapes such as an inverted triangle (in Japan's case).
Has the word "STOP" written in white letters under various languages.
Not to be confused with no entry sign, which in Japan is a red circle with a diagonal bar (Danbooru post #3627577 is an example).
This tag implicates road_sign (learn more).