The quality or state of being skilled and expert at doing something.
Adept (Latin adeptus) plus -ness (suffix forming abstract nouns). The suffix -ness transforms the adjective into a noun representing the quality itself.
Adeptness is what you build through repetition—musicians develop adeptness through thousands of hours of practice, and the word itself reflects that it's an attained quality, not something you're born with.
Complete word intelligence in one call. Free tier — 50 lookups/day.