A metric is a measure for quantitatively assessing, controlling or selecting a person, process, event, or institution, along with the procedures to carry out measurements and the procedures for the interpretation of the assessment in the light of previous or comparable assessments.
Metrics are usually specialized by the subject area, in which case they are valid only within a certain domain and cannot be directly benchmarked or interpreted outside it. This factor severely limits the applicability of metrics, for instance in comparing performance across domains. The prestige attached to them may be said to relate to a 'quantifiability fallacy', the erroneous belief that if a conclusion is reached by quantitative measurement, it must be vindicated, irrespective of what parameters or purpose the investigation is supposed to have.
In business, they are sometimes referred to as key performance indicators, such as overall equipment effectiveness, or key risk indicators. In the field of Facilities Management, a key metric is the Facility Condition Index, or FCI.
For a measure to be a metric it has to satisfy four properties: 1) non-negativity, 2) reflexivity, 3) symmetry, and 4) triangular inequality 
- S. Theodoridis and K. Koutroumbas, Pattern Recognition, Fourth Edition, Academic Press, 2009, p. 602.