Definition:
Great care and attention to detail.
Completeness, perfection.
Doing a thing so well that there is nothing left to be desired.
Completeness:
State or condition of being complete.
Perfection:
Quality of being perfect.
We can see the above image that the man is doing his work thoroughly and he is having thoroughness in his work.
With thoroughness he can succeed in life.
Precisely:
Exactly.
Examples:
Carefully painted chair to avoid getting paint on the floor with thoroughness.
Practiced dance presentation with thoroughness.