acid-base indicator(n = noun.substance) - an indicator that changes color on going from acidic to basic solutions;
is a kind of indicator
has particulars: phenolphthalein, congo red, methyl orange, litmus, litmus test, bromophenol blue, bromphenol blue, tetrabromo-phenolsulfonephthalein, bromothymol blue, bromthymol blue

absorption indicator, acid-base balance, acid-base equilibrium, flight indicator, fuel indicator, indicator, indicator lamp, leading indicator, oxidation-reduction indicator, speed indicator, torsion indicator, turn indicator