IPA: /ˈnaɪs.nəs/
KK: /ˈnaɪs.nəs/
The quality of being pleasant, kind, or agreeable to others.
Her niceness made everyone feel welcome at the party.
Niceness → It is formed from "nice" (from Latin *nescius*, meaning ignorant or unaware) and the suffix "-ness" (meaning the state or quality of). The word "niceness" refers to the state of being nice or pleasant.
Think of the quality of being 'nice' — the suffix '-ness' indicates a state, so niceness means the state of being nice.