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.
[2] The complexity of biological systems makes evaluating them a challenge.
A famous popular science individual psychological assessment is the Myers–Briggs Type Indicator.