A swastika (also more fully "cross gammadion", "gammadion cross"). Also: a figure composed of four capital Greek gammas placed back to back in such a way as to form a voided Greek cross.
gammadion/ɡəˈmeɪdɪən/ /ɡaˈmeɪdɪən/ /ɡəˈmadɪən/
Early 19th century. From (i) post-classical Latin gammadium, plural gammadia.