Merriam-Webster's Word of the Day for August 14, 2025 is immutable. Immutable is a formal adjective describing something that cannot be changed. For instance, the idea that cats and dogs are natural enemies is not immutable. Naturalists in the 1800s questioned the assumption that species were immutable. They proposed that organisms evolved and became more complex over time. Charles Darwin popularized these ideas, though he noted evolution's diverse paths. Many people, however, perceived evolution as a linear progression. Interestingly, the word "immutable" itself is mutable, capable of change. This word originated from the Latin verb "mutare," meaning "to change." Other words like mutate, transmute, and commute also stem from "mutare," highlighting language's inherent mutability.