The quality or state of being equable; calmness and evenness of temperament or consistency in conditions.
From equable + -ness (condition or quality of being). Equable derives from Latin aequabilis. The -ness suffix is Germanic and converts adjectives into abstract nouns.
The fact that English has both 'equability' and 'equableness' for the same concept shows how our language is beautifully redundant—sometimes we use Latinate suffixes, sometimes Germanic ones, depending on what just sounds better to our ear.
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