Indicator is a substance that changes color as conditions change; e.g., pH indicators reflect changes in acidity or alkalinity.
Definition 2.
Indicator refers to
1. In intelligence usage, an item of information which reflects the intention or capability of an adversary to adopt or reject a course of action.
2. In operations security usage, data derived from friendly detectable actions and open-source information that an adversary can interpret and piece together to reach conclusions or estimates of friendly intentions, capabilities, or activities.