An instrument (measurement, dataset, model, expert elicitation system) for quantifying an attribute, providing quantitative information of the system. For instance, the protocol for soil sampling and pH (KCL) measurement is an indicator for the ‘soil pH’, and the extraction, counting, identification of nematodes and calculation of the maturity index is an indicator for the ‘nematode community in the soil system’. Note that this definition differs from the daily practice where, for example, the pH or the nematode community as such, and not the protocol, is seen as the indicator.