oscuro ADJECTIVE dark
Oscuro is the opposite of claro in many contexts, and has the basic meaning of dark.
Oscuro can refer to the absence of light, either literally:
una habitación muy oscura a very dark room
or in a figurative sense:
la noche oscura del alma the dark night of the soul
Oscuro also refers to dark colors:
un traje gris o azul oscuro a dark grey or dark blue suit
Tiene el pelo castaño oscuro y grandes ojos oscuros. He has dark brown hair and big dark eyes.
The mysterious dark matter that astronomers believe makes up most of the mass in the universe is materia oscura in Spanish.