The quality or state of being stretched out, expanded, or made to reach across a wider area or longer time.
From 'extended' plus the noun-forming suffix '-ness', which turns adjectives into abstract nouns describing their qualities. This follows the Germanic pattern of creating nouns from adjectives.
The '-ness' suffix is so productive in English that you can technically add it to almost any adjective, creating words that may be rare but completely understandable—'extendedness' clearly means 'the quality of being extended,' even if nobody writes it often!
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