Originally: †incision into an abscess, tumour, etc. (obsolete). In later use: opening of the capsule of the lens in order to treat a cataract; an instance of this. Now chiefly historical.
Early 17th century (in an earlier sense). From post-classical Latin discission-, discissio action of tearing apart, separation, division, schism from classical Latin dīsciss-, past participial stem of dīscindere + -iō.