Sodium hydroxide

sodium hydroxide (common name caustic soda)
potassium hydroxide (potash lye)
lye (generic term, for either of the previous two, or even for a mixture)
sodium carbonate (soda ash)
potassium carbonate (potash) ...

Sodium hydroxide - highly active alkaline accelerator used in conjunction with hydroquinone to produce high contrast developers.
Sodium metabisulfite - used as an acidifying agent in acid fixing baths.

See SODIUM HYDROXIDE, SODIUM CARBONATE, SODIUM META- BORATE, and SODIUM BORATE. ACHROMATIC COLORS-Colors perceived as having no hue (white, black, gray, and silver). ACHROMATISM -The absence of chromatic aberration.

A chemical ingredient of film or paper developers that's often referred to as an activator and speeds up the developing agents. It's usally an alkali such as sodium carbonate, borax or sodium hydroxide.
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Aperture priority exposure ...

term used for that step in stripping or removing of an alkali soluble resist when an etched circuit board is dipped and soaked in a high strength (6-10%) solution of a sodium hydroxide, ...

Similarly a solution of potassium or sodium hydroxide can replace the Kodak S2 activator.

See also: Film, Alkali, Developer, Processing, Photograph