beberapa definisi yang bisa ditemui (fungsinya bisa ditemukan dalam definisi tersebut):

A  substance  that,  because  of  the  reactions  it  causes,  is  used  in  analysis  and  synthesis. 

A substance used in a chemical reaction to detect, measure, examine, or produce other.

A substance used (as in detecting or measuring a component, in preparing a product, or in developing photographs) because of its chemical or biological activity.

A substance or compound that is added to a system in order to bring about a chemical reaction or is added to see if a reaction occurs.