“Complex” derives from the Latin verb complector, which means to encircle, to embrace firmly and, metaphorically, to hold, to comprise, to unite, to re-unite under a single thought and a single denomination. Other meanings that appear in the Latin classics are those of bond, connection, concatenation.
From the XVII century onwards, a situation, a problem, a system is complex if it is made of interrelated parts influencing each others.
A complicated problem, on the other hand (from complico, to fold), is one which is hard to solve because it contains a great number of hidden parts, which must be revealed one after another but, still, can be treated separately because they do not interact.
Mutual interaction of components causes the nonlinear behavior of the whole. In principle, all “systems” are complex, and linearity is not but a technical approximation.
Notice that the number of parts/components is irrelevant in the definition of complexity: there can be complexity (nonlinear behavior) in systems as small as Poincaré’s 3-body problem or the prey-predator model.
When a system/problem is [regarded as] complex, the analytical approach (dividing into parts to make the problem tractable) is no longer sufficient and it must be complemented with a holistic, global approach to the problem as a whole.
 P.Magrassi, Difendersi dalla complessità, Franco Angeli, Milano 2009, pag. 130