Opacity describes how much color a layer of colored pencil blocks from passing through its surface.
An opaque color conceals what is underneath, we call this a masking pencil.
A transparent or translucent color allows the color below to remain visible, we call these glazing pencils.
A pencil may contain highly opaque pigments but still struggle to cover another color because it cannot adhere to the existing layers. Although in general, translucent/transparent pencils tend to be sticky and they adhere extremely well.
Opacity, adhesion, and pigmentation are related but separate characteristics.
In the Lab, we test opacity by applying colored pencil over a surface blackened by Copic Marker #100 to see how effectively the pencil conceals the black underneath.