The quality or state of having the same boundaries or limits; the condition of sharing identical extent or scope.
From coterminous plus the noun-forming suffix '-ness', which converts adjectives into abstract nouns describing the quality or state of being that adjective.
Adding '-ness' to adjectives is English's way of turning 'the quality of being [adjective]' into a single word—coterminousness means 'the state of sharing boundaries,' and it's only one word instead of six!
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