Each character in this phrase contributes to a part of the definition, which makes it very easy for you to learn! If we break it down, we can see that 火 (fire), 山 (mountain) and 口 (mouth) combine to literally mean “fire mountain’s mouth”, or “mouth of the fire mountain”. 火山 means volcano, and the mouth of the volcano is the crater at the top of the mountain where lava erupts! So, it’s very logical that the phrase 火山口 translates to the English word “crater”. Note that this only refers to volcanic craters, and not craters made from asteroids or other objects.
Pinyin: 火 huo3; 山 shan1; 口 kou3