z-scores describe how far a measurement is from the mean (average) or median; also referred to as standard deviation (SD) scores.
Source: Porta M, Greenland S, Hernán M, dos Santos Silva I, Last JM, ed. A dictionary of epidemiology, 6th edition. New York: Oxford University Press; 2014.
Last JM. A dictionary of epidemiology. 4th Edition. New York: Oxford University Press; 2001.