The variability of a process (within its natural limits) that is caused by many irregular and unpredictable (and individually insignificant) fluctuations or random factors that can not (practically) be anticipated, recognized, identified or eliminated.
“as incomplete information: simply what I cannot guess is random because my knowledge about the causes is incomplete, not necessarily because the process has truly unpredictable properties.”v
Taleb, N., N. (2010). The Black Swan: The Impact of the Highly Improbable (2nd Edition). New York: Random House Trade Paperbacks. (p. 308)