I John 2:16 Amp
For all that is in the world, the lust of the flesh [craving for sensual gratification], and the lust of the eyes [greedy longings of the mind] and the pride of life [assurance of one’s own resources or in the stability of earthly things] — these do not come from the Father but are from the world [itself].

Even though this verse still doesn’t exactly answer for me “what is ‘the world’?,” it does help to show what being stained by the world looks like in us. The next problem (which does have an answer) is: what do we do about it?