Where is the line drawn with stealing?
‘If a monk, intending to steal, takes from a village or from the
  wilderness what has not been given to him— the sort of theft for which
  kings, having caught a thief, would beat, imprison, or banish him,
  saying, “You’re a bandit, you’re a fool, you’ve gone astray, you’re a
  thief”— he too is expelled and not in communion.’ Second Parajika rule 


This is for monks, but it's pretty clear - or at least it leaves the subjectivity to individual jurisdictions! (so in Italy, if you are starving and steal food, it's ok. If it's less than the value of a good shield (1/4 dinar), then in Sharia countries you are not in trouble)

With regards to possession, who possesses and to what extent, are probably useful measures. Who possesses is usually well defined (if not, then theft becomes a gray area). To what extent - how long should patents and copyright last, whether you can even effectively charge for something like streetlighting - are all likely subject to a social consensus. 

Overall, if you perceive the object of your desire to be in the possession of someone else, or that someone else directly tells you so (asking is always a nice way to establish possession), then any intentional act to take such a thing, against the wishes of the owner would be theft, regardless of social consensus. 

Piracy and claims as to the injustice of any and all economic systems are ancient claims, modernity only offers a novel spin on them.

If you think/believe/know that within a jurisdiction, a thing is claimed by someone and they don't have any desire to relinquish it, intentional taking of it is theft. Whether that possession is rightfully in their hands or not, is for their kamma, not yours.