Opacity is the characteristic of being difficult to understand or unclear. A professor whose lectures are known for opacity is going to find himself in a room full of some very frustrated students.

In its literal sense opacity is used to talk about objects that don't allow light to shine through. A window that has accumulated a lot of dirt and dust is characterized by its opacity, as opposed to a transparent window, which is one you can see right through. The word opacity is used more generally for anything obscure, unclear, or incomprehensible, like the opacity of a novel full of ten-syllable words or a dictionary that doesn't give you any clear or straightforward definitions.

Definitions of opacity

n the quality of being opaque to a degree; the degree to which something reduces the passage of light

clarity, clearness, uncloudedness
the quality of clear water
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cloudiness, muddiness, murkiness
the quality of being cloudy
blurriness, fogginess, fuzziness, indistinctness, softness
the quality of being indistinct and without sharp outlines
turbidity, turbidness
muddiness created by stirring up sediment or having foreign particles suspended
haziness, mistiness, steaminess, vaporousness, vapourousness
cloudiness resulting from haze or mist or vapor
dimness, faintness
the quality of being dim or lacking contrast
indistinctness of shape or character
Type of:
an essential and distinguishing attribute of something or someone

n the phenomenon of not permitting the passage of electromagnetic radiation

transparence, transparency
permitting the free passage of electromagnetic radiation
optical opacity
opacity to light
radio-opacity, radiopacity
opacity to X-rays or other radiation
Type of:
physical phenomenon
a natural phenomenon involving the physical properties of matter and energy

n incomprehensibility resulting from obscurity of meaning

Type of:
the quality of being incomprehensible

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