Plasma

https://plasma.io/
https://plasma.io/plasma.pdf

Plasma is a system of smart contracts that allows implementation of (hierarchy of) side blockchains, that are validated as a whole as a main chain. The benefit is that the computation of side contracts can be conducted "off-chain", with the minimal computation hitting the main chain. Such multichain structure reduces burden on the main chain, and hence can potentially lead to a dramatic increase of the transaction processing power, to billions per second, in necessary.

Each side block chain can be internally validated with much lower computing power (in case of Proof of Work) and hence lower validation fees.