The quality or state of being delightful; the characteristic of bringing joy, pleasure, or great enjoyment.
From delightful (delight plus -ful, meaning 'full of') plus the abstract noun suffix -ness, creating a noun describing the quality of being full of delight.
Notice how 'delightfulness' uses -ful and -ness together, while 'delightedness' uses -ed and -ness—different base words (adjective vs. past participle) create different nuances, showing how English builds infinite shades of meaning from limited building blocks.
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