Hash Market
A market for renting "Hash Power". It buys and/or sells hash power normally used for bitcoin mining. The price of hash power is often more than its "worth in bitcoin" (= the amount of bitcoin it is expected to generate).
One example of such a marketplace was GPUMAX, ran by Pirateat40.
Hash markets can be used to obtain large amounts of hash power, in order to perform various attacks that require high hash rate against some alt-chains (or theoretically bitcoin itself).