The number of simultaneous or near- simultaneous interactions of trajectories in a given volume of airspace.
SESAR Concept of Operations Step 2 Edition 2014 (Ed. 01.01.00)
Given complexity definition refers to ATM context.
For automation, complexity is relevant only in terms of calculation effort, not the ability to solve a given set of problems. Beyond a certain level of complexity, humans can no longer oversee all the consequences of the interactions and automation support is required if traffic is to be handled safely and efficiently. See also Density.
Lexicon Release 2015.1