In the widest sense, an indicator is a sign or a signal that can convey a message, The message conveyed may be simple or it may be complex. Indicators provide us with information on things, situations, activities, processes or phenomena that exist or that are occurring in our surroundings. According to McQueen and Noack^[1], indicators are defined as “. . .measures that summarize information relevant to a particular situation, or a reasonable proxy for such a measure”.

Examples of indicators include:

• A directional signal of an automobile: It can indicate when and how an automobile will turn.
• The temperature reading on a thermometer:
  • It can indicate indoor or outdoor comfort levels;
  • It can indicate the likelihood of disease-related conditions.

Indicators can have their greatest value as proxies or substitutes for measuring conditions that are so complex that currently there are limited possibilities for direct measurement. For example, data from reconnaissance orbiter photographs—and such other indirect measurements as exploration rover-based spectrographs taken recently—of the surface of Mars indicate the likely past presence of water.

Also, indicators may be aggregated or grouped (with or without weighting) into what are referred to as either indices or indexes. Often, these indices or indexes are useful in conveying complicated information in a simple, straightforward manner. A good example of an index is The American Consumer Satisfaction Index: a conglomeration—or aggregation—of econometric data collected through interviews. It is used to indicate how comfortable consumers say they feel about their economic condition—and that of the U.S. economy.

Indicators are developed and used predominantly to highlight the workings or the performance—or the lack thereof—of a system. The system may be biological, physical, chemical, economic or social. These indicators can tell us something about a system's status—and over time, about a system's operating trends. Indicators are used worldwide by scientists, governments, private-sector entities, and organizations and individuals in the general public. Their use of indicators boils down to having a “need-to-know”.

Again, indicators are as varied as the types of systems they monitor. However, according to the Website of Sustainable Measures, a provider of sustainability training services, there are certain characteristics that useful, effective, defensible and believable indicators have in common:

• Effective indicators are relevant; they show you something about the system that you need to know.
• Effective indicators are easy to understand, even by people who are not experts.
• Effective indicators are reliable; you can trust the information that the indicator is providing.
• Effective indicators are based on accessible data; the information is available or can be gathered while there is still time to act.