Answer: An indicator is a device or an instrument that is used to measure or record variations pertaining to a solution or substance.
Two good properties of an indicator are as follows-
1) It should be specific- It means that the changes, which are measured by the indicator should be expressed in precise terms so that appropriate assessment could be made.
2) It should be time bound- This means that the measurable changes should arrive withing a particular time limit.
Example of indicator- Litmus is an acid base indicator that turns acidic solutions red and basic solutions into blue color.