Proof of Work (PoW) doesn't only relate to Bitcoin or other systems involving Block Chain Technology.
Quote from Cryptography Wiki on Wikia 
A Proof-of-work ("POW") system (or protocol, or function) is an economic measure to deter denial of service attacks and other service abuses such as spams on a network by requiring some work from the service requester, usually meaning processing time by a computer. A key feature of these schemes is their dissymmetry: the work must be moderately hard (hard but feasible) on the requester side but easy to check for the service provider. This idea is also named Client Puzzle Protocol (CPP). It is distinct from a CAPTCHA which is intended to be solved quickly by a human, and not a computer.