Lye or caustic soda is the trade name for sodium hydroxide, hence 50% caustic soda is an aqueous solution containing between 48 and 50% by weight of NaOH.

It has a wide range of applications, with the main ones being the production of oils, soaps, and detergents; regeneration of ion exchange resins for water treatment; glass bottle washing; bleaching of pulp in the pulp and paper industry; refined sugar production; cotton fabric dyeing; production of agrochemicals; manufacture of gelatin and jellies; general cleaning products, etc.

Caustic soda is transported in tanker truck tanks made of carbon steel or railroad tank cars of the same material.

Because caustic soda is highly corrosive, it attacks leather, wool, rubber, phenol-formaldehyde resins, and many metals such as aluminum, magnesium, zinc, lead, copper, tin, and bronze, and galvanized surfaces.