Problems which are at least as hard as the hardest problems in NP.

NP-Complete problems are a subset of NP-hard problems.


Reduction using a turing machine (no idea how)

Indirect reduction from existing NP problems.

We can do this because we have already proved other problems in NP-hard reduce to these problems. As such we can just reduce this problems to a new problem we have, to establish the new problem is NP-hard.