1. c. gen. rei et dat. pers., being the joint or contributory cause of a thing which benefits another, ς. τινὶ ἀθανασίας helping him towards . . , Isoc.5.33, cf. ib.44; εἰ τῶν πεπραγμένων ἐκείνοις μηδὲν ς. ἐγενόμην if I had contributed nothing to their success, Id.15.96; “πᾶσιν ἀγαθοῦ ς. ὤν” X.Cyr.1.4.15, etc.
2. c. gen. rei only, being joint-cause of, accessory to, contributing to, Pl. Plt.281c, Grg.519b, D.18.64, Thphr.Sens.27, etc.; ς. τούτου συνειπὼν Φιλοκράτει by acting as his advocate, D.19.97: as Subst., ἡ ξυναιτία φόνου accomplice in murder, A.Ag.1116 (lyr.); “δοξάζεται . . οὐ συναίτια, ἀλλ᾽ αἴτια εἶναι τῶν πάντων” Pl.Ti.46d.