1. A thick or dark condition that makes something; such as, glass or a liquid difficult to see through: The opacity of Shareen's windows were a result of not having the accumulation of dirt cleaned off.

Opacities represent the degrees to which light is obscured by pollutant particles in the air.

2. Having difficulty in understanding statements, presentations, or in explaining something: Students had difficulty in understanding the opacity of the teacher's scientific presentation.

The opacities of most dictionaries simply don't give people clear or straightforward definitions.

3. Etymology: "darkness, obscurity", from French opacité; from Latin opacitatem, opacitas, "shade, shadiness"; from opacus, "shaded, dark".

1. The state of being opaque (not allowing the passage of light); unperceptive, dense: Somehow the opacity of Joe's glasses was due to the lenses being smeared over and dirty.
2. On a radiograph, the phenomenon of not allowing the passage of electromagnetic radiation: Dr. Simpson complained about the opacity that the x-rays showed and wanted to have the process repeated.
3. Something that is obscure or incomprehensible: The authors hope that tbis dictionary is not characterized by opacity, but has clear and understandable definitions and example sentences.

Jim's math teacher was known for her opacity and his friends were all very frustrated, even after asking specific questions about the content of the lessons.