complexitate v angličtině:

Many things combine to present human experience in all its complexity.

I would like to draw your attention to the complexity of the current situation.

We have to make sure we understand the complexity of the matter at hand.

complexity of a problem

It cannot plausibly be argued that behaviour of such complexity derives entirely from instinct.

However, I recommend that you take the JR Narita Express (a train leaves every 30-60 minutes from the airport), to avoid the complexity of changing trains.