Always happening in the same way; without changing or failing.
From *in-* “not” and *variable* “changing,” based on Latin *variabilis* from *variare* “to change.” It means “not variable,” so it never changes.
When something invariably happens, it’s so predictable you could set your watch by it. Notice how the word hides the math idea of a “variable”—here, it’s the thing that never acts like one.
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