IPA: /əˈsɪdʒuəsnəs/
KK: /əˈsɪdʒuəsnəs/
The quality of being diligent and hardworking, showing persistent effort and care in tasks or duties.
Her assiduousness in studying for the exam paid off with excellent results.
Assiduousness is formed from "assiduus" (from Latin, meaning sitting near or attending to) and the suffix "-ness" (meaning the state or quality of). The word describes the quality of being diligent and attentive in one's work or duties.
Think of someone who is 'attending to' their tasks with great care and diligence — that's what assiduousness means.