What is Application Blockchain Interface (ABCI)?

The Application Blockchain Interface (ABCI) is a specialized application programming interface (API) built by Tendermint, the company that created the Cosmos blockchain. The ABCI is designed to operate as a middle layer that allows blockchain-based replication engines present on several computers and a deterministic state machine (the application) present on a single computer to communicate. In simpler terms, the ABCI allows a blockchain protocol to communicate with an application to enable application development and other related purposes.