What is the Interledger Protocol (ILP)?

The Interledger Protocol is a currency-agnostic payments protocol that was developed by co-creators Stefan Thomas and Evan Schwartz in 2015. They modeled ILP to closely resemble the TCP/IP protocol suite that powers the internet today. Interledger can be integrated with any type of accounting ledger, including fiat and cryptocurrencies.

A more technical document exists here: https://interledger.org/developers/rfcs/interledger-protocol/.