any of various substances, such as litmus or phenolphthalein that indicate the presence, absence, or concentration of another substance or the degree of reaction between two or more substances by means of a characteristic change, especially in colour.

enige van verskeie stowwe, soos lakmoes of fenolftaleïne wat die teenwoordigheid, afwesigheid, of konsentrasie van'n ander stof, of die graad van reaksie tussen twee of meer stowwe deur middel van 'n karakteristieke verandering, gewoonlik in kleur, aantoon.