Opacity (pronounced "o-PASS-ity") refers to the opaqueness of an object. In computing, opacity is often used in graphics software to define how "nontransparent" an image is. In other words, the more transparent an image or image layer is, the lower its opacity.
Adobe Photoshop and other graphics editors provide an Opacity slider that you can use to adjust the opacity of an image layer. Typically, this setting ranges from 0 to 100. Setting the opacity to 0 makes the layer completely transparent, or invisible. Setting it to 50 makes it half transparent or "see-through." If you give a layer an opacity of 100 (usually the default setting), it will be completely opaque. In a multi-layer image, all layers underneath a layer that fills the canvas with an 100% opacity will be completely hidden.
Updated: March 26, 2013