noun
constancy; invariability; permanence
Abstract noun referring to the quality of being constant, unchanging, or permanent. Often used in philosophical, scientific, or formal contexts.
自然法則の恒性は科学の前提である。
The constancy of natural laws is a premise of science.
彼の愛情の恒性には感心する。
I admire the constancy of his affection.
Compound of 恒 (constant, always) and 性 (nature, quality). The exact historical derivation is uncertain, but the word follows a common Sino-Japanese pattern for abstract nouns.