noun
A lake formed in a volcanic crater, typically within a caldera or on a volcano's summit.
火口原湖の水は、しばしば強い酸性を示す。
The water of a crater lake often shows strong acidity.
カルデラ湖 is a caldera lake, which is a broader term for a lake in a large volcanic depression, while 火口原湖 specifically refers to a lake within a crater.
Compound of 火口 (crater) + 原 (field, plain) + 湖 (lake), literally 'crater-plain lake'. The exact historical derivation is uncertain, but the term is used in volcanology.