The quality or state of being dense; the degree to which something is closely packed or thick.
From dense + -ness (suffix forming abstract nouns). This is a straightforward formation: dense (adjective) + -ness = denseness (abstract noun expressing the quality).
The 'denseness' of a forest determines what you can see—a dense rainforest is so thick that sunlight barely reaches the ground, creating an entirely different ecosystem than an open woodland with the same number of trees.
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