The quality of remaining calm, composed, and unperturbed regardless of circumstances.
From 'equanimous' plus '-ness,' though 'equanimity' (from Latin 'aequanimitas') is the more standard noun form. This variant emphasizes the state of being equanimous.
English has both 'equanimity' and 'equanimousness' for the same concept—a redundancy showing how older Latin loans compete with Germanic suffixes in English even for the same ideas.
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