How much a material blocks light from passing through — fully opaque means no light gets through at all.
Opacity is the inverse of transparency; a fully opaque surface transmits none of the incident light, reflecting or absorbing all of it instead.
Opacity (light transmission) is a different property from finish (matt/gloss, which is about surface reflection), though both affect how a colour reads.
A solid emulsion wall paint is opaque; a coloured glass pane is not.
Paint opacity/coverage affects how many coats are needed and how much an existing wall colour shows through.
Paint and paper opacity are measured as a contrast ratio: the coating is applied over a chart that is half black and half white, and its reflectance over the black is divided by its reflectance over the white. A result near 100% means the background no longer shows through. Hiding-power calculations commonly take 98% as the target, and the film thickness needed to reach it is what differs so much between colours. In CSS and image editing the word means something else, the alpha value from 0 to 1.
A deep red wall is repainted in a pale yellow or a clean white.
Pale and bright colours built on weakly scattering pigments let the old colour through for coat after coat. A grey or tinted undercoat that hides well does the work the finish coat cannot.
A logo is printed in a light ink on dark card or clear film.
Process inks are transparent by design, so the substrate shows through and the colour disappears. The job needs an opaque white layer under the colour, or an opaque spot ink.