[in.di.ca.tor] n (1666) 1: one that indicates: as a: an index hand (as on a dial): pointer b (1): gauge 2b, dial 4a (2): an instrument for automatically making a diagram that indicates the pressure in and volume of the working fluid of an engine throughout the cycle
2. a: a substance (as litmus) used to show visually (as by change of color) the condition of a solution with respect to the presence of a particular material (as a free acid or alkali) b: tracer 4b
3: an organism or ecological community so strictly associated with particular environmental conditions that its presence is indicative of the existence of these conditions
4: any of a group of statistical values (as level of employment) that taken together give an indication of the health of the economy -- in.dic.a.to.ry adj