The quality or state of being cathartical; the capacity to produce emotional or spiritual cleansing.
From cathartical + -ness (noun suffix), an archaic noun form. The accumulation of suffixes shows how English creates abstract qualities from Greek medical roots.
This word is so archaic and suffix-heavy that you almost never see it used, but it's a perfect example of how English-speakers used to love stacking suffixes to create incredibly specific philosophical and medical terms.
Complete word intelligence in one call. Free tier — 50 lookups/day.