Real Dictionary

complex
a characteristic feature or quality distinguishing a particular person or thing [...]
A complex of things is a group or system of things that are connected with each other in a complicated way.
a whole made up of interconnected or related parts [...]
any impairment of normal physiological function affecting all or part of an organism, esp a specific pathological change caused by infection, stress, etc, producing characteristic symptoms; illness or sickness in general [...]
In grammar, a complex sentence contains one or more subordinate clauses as well as a main clause. Compare compound, , simple.
adjective: Something that is complex has many different parts, and is therefore often difficult to understand.
countable noun: A complex is a group of buildings designed for a particular purpose, or one large building divided into several smaller areas.