  There is no single definition of complexity (Edmonds 1999; Gershenson 2008;
Mitchell 2009; De Domenico, et al., 2019), as it acquires different meanings in
different contexts. A general notion is the amount of information required to
describe a phenomenon (Prokopenko, et al. 2008) , but it can also be understood
as the length of the shortest program required to compute that description, as
the time required to compute that description, as the minimal model to
statistically describe a phenomenon, etc.
