noun
permanent and immutable; eternally unchanging
Formal, often used in philosophical, legal, or rhetorical contexts to describe principles, truths, or conditions that are absolutely unalterable.
この憲法の基本理念は恒久不変である。
The fundamental principles of this constitution are permanent and immutable.
彼は恒久不変の真理を探求した。
He sought eternal and unchanging truth.
不変 means 'unchanging' or 'invariable' and is more common in everyday contexts. 恒久不変 adds the nuance of 'eternal' or 'perpetual' and is more emphatic and formal.
永遠 means 'eternity' or 'forever' and is more commonly used. 恒久不変 emphasizes not just duration but also the quality of being unchangeable.
Compound of 恒久 (eternal, perpetual) and 不変 (unchanging, immutable). The exact historical derivation is uncertain, but the word is a Sino-Japanese compound used in formal writing.