The quality or state of being compound; the characteristic of being made up of multiple parts or elements.
From compound (Latin com- + ponere) plus the abstract noun suffix -ness, which creates nouns describing qualities. This formation became common in English around the 15th-16th centuries.
The compoundness of water molecules—the fact that hydrogen and oxygen combine—was revolutionary when scientists realized that substances could be made of distinct elements bound together, fundamentally changing our understanding of matter!
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