invariable; permanent; constant and unchanging
Describes something that is fixed and does not change over time; often used in formal or written contexts.
The laws of nature are not invariable.
His beliefs are constant and unchanging.
不変 simply means 'unchanging' or 'immutable', while 一定不変 emphasizes that something is both fixed (一定) and unchanging (不変), often implying a stronger sense of permanence.
恒常 also means 'constant' or 'permanent', but is more technical and often used in scientific or philosophical contexts, whereas 一定不変 is more general and can be used in everyday formal writing.
Compound of 一定 (いってい, 'fixed, constant') and 不変 (ふへん, 'unchanging'). The combination reinforces the idea of something that is both set and immutable.