IPA: /ˈkɛr.fəl.nəs/
KK: /ˈkɛrflnəs/
The quality of being cautious and paying attention to details to avoid mistakes or accidents.
Her carefulness in planning the event ensured that everything went smoothly.
Carefulness → It is formed from "care" (from Old English "caru", meaning worry or concern) and "-ful" (meaning full of) and "-ness" (meaning the state of being). The word "carefulness" means the state of being full of care or concern.
Think of being 'full of concern' ('care') and in a 'state of being' ('-ness') — that's what carefulness means.