Some substances whose odour changes in acidic or basic media are called ________ indicators. 

An Olfactory indicator is a substance whose smell varies when mixed with an acidic or basic solution. Such substances can be used in the laboratory to test whether a solution is a base or an acid, and this process is called olfactory titration. All onion, clove oil and vanilla extract are examples of olfactory indicators.

Hence, Some substances whose odour changes in acidic or basic media are called olfactory indicators.