The quality or state of being compatible; the degree to which things can coexist or function together.
From compatible + -ness suffix (Old English origin, forming abstract nouns). This is a doublet of compatibility, using the Germanic -ness instead of the Latinate -ity.
English has an embarrassment of riches with words like 'compatibility,' 'compatibleness,' and 'compatibly'—we can express the same idea three different ways using different suffix families, a quirk that makes English both flexible and redundantly complex.
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