Opacity is used to describe how much light can pass through an object ranging from transparent through translucent to opaque. A substance that is translucent lets some light pass through making it partially see-through whilst an object or substance that is transparent lets a lot of light through, giving the appearance of it being totally see-through. The term opaque originated from the Latin, meaning ‘dark’ meaning ‘not transparent’ and opaque substance does not let any light pass through at all.
A paint that is opaque will give a solid colour. Blacks and whites are always opaque and any colour mixed with them will become more opaque. Gouache, for instance, is Watercolour with white added to it. This has the result of giving Gouache Paint a flat, solid colour as opposed to Watercolour that uses its translucency to let light through the colour and bounce off the white paper surface. Oil and Acrylic colours can be either translucent or opaque depending on how the pigments used react with the binder. This is dependent on the colour rather than the range so any given range will have both opaque and translucent colours in it.