In statistics and research design, an indicator is an observed value of a variable, or in other words "a sign of a presence or absence of the concept being studied".[1] Just like each color indicates in a traffic lights the change in the movement.
For example, if a variable is religiosity, and a unit of analysis is an individual, then that one of potentially more numerous indicators of that individual's religiosity would be whether they attend religious services; others - how often, or whether they donate money to religious organizations.[1]
Numerous indicators can be aggregated into an index.[2]
↑Hawken, Angela; Munck, Gerardo L. (24 April 2012). "Cross-National Indices with Gender-Differentiated Data: What Do They Measure? How Valid Are They?". Social Indicators Research. 111 (3): 801–838. doi:10.1007/s11205-012-0035-7.