In chemistry, to bleach something generally means to whiten it or oxidize it. A bleach is a chemical that exhibits these properties. Common chemical bleaches include sodium hypochlorite, or "chlorine bleach," and "oxygen bleach," which contains hydrogen peroxide. "Bleaching powder" is calcium hypochlorite . Bleaching can be a preliminary step in the process of dyeing.
Chlorine bleach is commonly used as a disinfectant by homemakers and janitors. Bleach should NEVER be mixed with cleaners containing ammonia; the mixture can create toxic gases and volatile explosives.
In photography
In most color negative processes, the silver halide crystals present in the emulsion are removed using chemical bleaches. (unlike black and white negatives, which contain silver in all the dark areas). Photographic bleach is usually potassium ferricyanide.