adjective

1. not letting light pass through; not transparent or translucent
2. not reflecting light; not shining or lustrous; dull or dark
3. not allowing electricity, heat, etc. to pass through
4. hard to understand; obscure
5. slow in understanding; obtuse

Etymology: ME opake < L opacus, shady

noun

1. anything opaque
2. Photog. an opaque liquid used in blocking out parts of a negative

transitive verb opaqued opaqued′, opaquing opaqu′·ing

1. to make opaque
2. Photog. to apply opaque to (a negative)