Meaning of “complex” - Learner’s Dictionary

complex

adjective us uk /ˈkɒmpleks, kəmˈpleks/
It's a very complex issue.The book conveys a complex message.The whole system seems to me needlessly complex.The building has a very complex structure.Historians have tried to untangle the complex issues behind the events.

B2 involving a lot of different but connected parts in a way that is difficult to understand:

complex details/issues
The situation is very complex.
complexity noun [ C, U ] us uk /kəmˈpleksəti/

the quality of being complex:

the complexities of life

(Definition of “complex adjective” from the Cambridge Learner’s Dictionary © Cambridge University Press)