Lightning Network
The Lightning Network is a "second layer" payment protocol that operates on top of a blockchain (most commonly Bitcoin). It theoretically enables fast transactions between participating nodes and has been touted as a solution to the bitcoin scalability problem. It features a peer-to-peer system for making micropayments of digital cryptocurrency through a network of bidirectional payment channels without delegating custody of funds.
Normal use of the Lightning Network consists of opening a payment channel by committing a funding transaction to the relevant blockchain, followed by making any number of Lightning transactions that update the tentative distribution of the channel's funds without broadcasting to the blockchain, optionally followed by closing the payment channel by broadcasting the final version of the transaction to distribute the channel's funds.