The "one wing" refers to the title of Safer Sephiroth's battle theme "One-Winged Angel," which is not a literal translation of the original Japanese title - which would be "Angel With A Wing On One Side," obviously referring to the one black wing Safer Sephiroth had for an arm. The appearance of a single black wing (as opposed to a single black wing plus six white ones) in Advent Children, Crisis Core and Dissidia is a reference to the English title of "One-Winged Angel."