According to the EEA, an indicator is a measure, generally quantitative, that can be used to illustrate and communicate complex phenomena simply, including trends and progress over time. – E.g.: Many success factors are more or less complex phenomena that can be expressed in general terms (e.g., peripherality, transport links, etc.) or are represented by nominal (non-numerical) string variables (e.g., previous use, zone of location, etc.). Indicators represent simplifications and quantifications of factors into measurable variables (e.g. peripherality ® distance [km] from regional centre). Usually one factor can be measured via more alternative or complementary indicators. The selection of specific indicators is determined mostly by the availability of data.

 ‘An indicator provides a clue to a matter of larger significance or makes perceptible a trend or phenomenon that is not immediately detectable. An indicator is a sign or symptom that makes something known with a reasonable degree of certainty. An indicator reveals, gives evidence, and its significance extends beyond what is actually measured to a larger phenomenon of interest’ (IETF, 1996).