When used as an adjective, opaque is descriptive of any substance or material that does not allow the transmission of light through it, or possesses a high degree of opacity. The opposite of opaque is transparent. A middle-ground is a translucent substance or material, which allows some light to pass through it, albeit diffusely.
The term opaque, when used as a verb, means to apply a thick fluid to portions of a photographic negative prior to platemaking, as a means of eliminating unwanted images or specks, known as opaquing or spotting out.
The term opaque, when used as a noun, refers to the thick, typically red or black fluid used for blocking out the aforementioned undesirable specks or images on photographic negatives. Also known as opaquing fluid. A special red felt-tip markers used for the same purpose is also called opaque.