A chemical or substance that initiates or accelerates a chemical reaction ‘s rate when added to another chemical or substance. The catalyst is unaltered in the process and is reusable until effectiveness is lost. An example is nickel that is the catalyst for the hydrogenation or solidification of vegetable oil. Another example is chlorophyll in plants is the catalyst for photosynthesis , converting carbon dioxide with water into sugars. Is the opposite of an inhibitor.