A host bus adapter (HBA) is a circuit board and/or integrated circuit adapter that provides input/output (I/O) processing and physical connectivity between a server and a storage device. Because the HBA relieves the host microprocessor of both data storage and retrieval tasks, it can improve the server’s performance time. An HBA and its associated disk subsystems are sometimes referred to as a disk channel.

It looks something like this:

In one of the used cases, it looks like,