Immutable is a formal adjective used to describe something that is unable to be changed.

// It is hardly an immutable fact that cats and dogs are sworn enemies; over the years our golden retriever has grown both fond and protective of her tabby housemate.

See the entry >

“... by the 1800s, naturalists like Lamarck were questioning the assumption that species were immutable; they suggested that over time organisms actually grew more complex, with the human species as the pinnacle of the process. Darwin brought these speculations into public consciousness in 1859 with On the Origin of Species, and while he emphasized that evolution branches in many directions without any predetermined goal in mind, most people came to think of evolution as a linear progression.” — Ted Chiang, LitHub.com, 6 Mar. 2025

Immutable may describe something that is incapable of change, but the word itself—like all words—is mutable, both capable of and prone to alteration. To put a finer point on it, if language were fixed, we wouldn’t have immutable itself, which required a variety of mutations of the Latin verb mutare (“to change”) to reach our tongues (or pens, keyboards, or touchscreens—oh the many permutations of communication!). Other English words that can be traced back to mutare include mutate, transmute, and commute. Which reminds us—the mutability of language makes great food for thought during one’s commute.