NP-complete
An NP-complete problem is an NP problem such that if one could find answers to that problem in polynomial number of steps, one could also find answers to all NP problems in polynomial number of steps. This makes NP-complete decision problems the hardest problems in NP (they are NP-hard). People spent lots of time looking for algorithms that finds answers to some NP-complete problem in polynomial number of steps, but have not found any. Because of this, if someone shows a problem to be NP-complete, it is not likely that there is an algorithm solving it in polynomial number of steps.