IPA: /səˈsɛs.fəl.nəs/
KK: /səˈsɛs.fəl.nəs/
The state of achieving desired goals or outcomes.
Her successfulness in the project earned her a promotion.
Successfulness → It is formed from "success" (from Latin "successus", meaning outcome or result) and "-ful" (meaning full of) and "-ness" (meaning the state of being). The word "successfulness" means the state of being full of success or achieving favorable outcomes.
Think of being 'full of' ('-ful') 'outcomes' ('success') — that's why successfulness means the state of achieving success.