This is the 18031st most frequent Korean word.
수성
"수성" means "Mercury" (the planet) or "water-based reform" depending on context.
In this sentence, '수성' refers to Mercury, a planet in the solar system.
수성은 태양계의 가장 작은 행성이다.
Mercury is the smallest planet in the solar system.
Here, '수성' is used to denote the planet that ancient astronomers studied.
고대 천문학자들은 수성을 관찰하여 많은 것을 배웠다.
Ancient astronomers learned a lot by observing Mercury.
In this usage, '수성' describes ink that is water-soluble or water-based.