IPA: /ˈθɜːrə(n)əs/
KK: /ˈθɜroʊnəs/
The quality of being very careful and paying attention to every detail in a task or process.
Her thoroughness in preparing the report impressed her manager.
Thoroughness → It is formed from "thorough" (from Old English *þurh*, meaning through or by means of) and "-ness" (a suffix indicating a state or quality). The word "thoroughness" refers to the quality of being thorough or complete in detail.
Think of being 'through' ('thorough') in your work, which means you cover everything completely. The '-ness' shows that it's a quality of being thorough.