In crafting with paper, glass or even paints and stains, opaque is a term used to describe the material's ability to block light. Opaque means light cannot travel through the object; in other words, it is neither translucent nor transparent. Opaque properties may apply to any color of crafting material.
Opaque, Transparent and Translucent
To understand opaque qualities, compare red construction paper and red tissue paper. The construction paper is opaque -- you cannot see through it at all -- while the tissue paper is translucent, meaning light travels through it. Holding the object up to a light source allows you to test whether it is translucent or opaque. A transparent item is virtually clear, such as a clear gift-wrapping tape. Paint is generally opaque when applied to a surface in a thick coat. Adding a transparent glaze allows you to see the color underneath. This technique comes in handy when creating a color wash, for instance, by layering a color such as a blue glaze over an opaque color, such as yellow.