-ness

turns an adjective into a noun naming its state, condition, or quality

Spelled -ness · -nes/-ness 'state, quality, or condition'

Old English -nes named a state: kindness is being kind, darkness is being dark, and readiness is being ready.

Words built from it

kindness — the quality of being caring and helpful
kind + -ness (state or quality)
darkness — the condition of having little or no light
dark + -ness (state or condition)
readiness — the state of being prepared
ready + -ness (state or condition), with y changing to i
awareness — knowledge or recognition of something
aware + -ness (state or condition)
stubbornness — an unwillingness to change one's mind
stubborn + -ness (state or quality)
loneliness — sadness caused by feeling alone
lonely + -ness (state or condition), with y changing to i

harness — It comes through French from Old Norse and originally meant equipment or armor; its ending is not -ness.
lioness — It is lion + -ess, the suffix for a female; the final n belongs to lion, so it does not contain -ness as a suffix.

-less — turns a noun into an adjective meaning without or lacking it
-ment — turns a verb into a noun naming the action, result, or means
-ology — turns a subject into the name of its study or body of knowledge
-ous — turns a noun or root into an adjective meaning full of, having, or characterized by it
-phobia/-phile — forms names for intense fears or aversions, or for people and feelings marked by strong fondness