Basic substance of a chemical reaction. In Plasmas technology the precursor is a gas or liquid, which is transformed into a gas by an evaporator. In plasma technology the precurser is the process gas in the recipient, which is stimulated by the plasma and transormed into ions, radicals and stimulated atoms. All of these spezies are very reactive. Therefore the precursor generates etching of many materials or is the base for polymerisation processes.