opacity n.

1550s, "darkness of meaning, obscurity," from French opacité, from Latin opacitatem (nominative opacitas) "shade, shadiness," from opacus "shaded, dark, opaque," a word of unknown origin. The literal sense "condition of being impervious to light; quality of a body which renders it impervious to rays of light" in English is recorded from 1630s.