What is "sulphion"

Sulphion \Sul"phi*on\, n. [Sulpho- + ion.] (Chem.) A hypothetical radical, SO4, regarded as forming the acid or negative constituent of sulphuric acid and the sulphates in electrolytic decomposition; -- so called in accordance with the binary theory of salts. [Written also sulphione.]

n. (context chemistry English) A hypothetical radical, SO₄, regarded as forming the acid or negative constituent of sulphuric acid and the sulphates in electrolytic decomposition.