A problem Q can be reduced to another problem Q' if any instance of Q can be ``easily rephrased'' as an instance of Q', whose solution provides a solution to the instance of Q.
Example: Solving ax + b = 0 reduces to solving .
A language L1 is poly-time reducible to language L2, written , if there exists a poly-time computable function such that for all x {0,1}*:
where f is the reduction function.
This is a one-way function. Q' will not always reduce to Q.