Also known as: DB2/400
The relational database built directly into the IBM i operating system, rather than installed as a separate product.
DB2 for i is the relational database management system built into IBM i itself, a defining feature of the platform since the original AS/400. Unlike most database systems, it isn't a separate product installed on top of the OS, it's part of the operating system, which is part of why IBM i systems have historically needed less dedicated database administration than comparable platforms.
DB2 for i supports both traditional native file access (used by older RPG programs) and standard SQL access side by side against the same physical data, which matters directly for migration planning: legacy programs and modern SQL-based tools can read and write the same tables without a data conversion step.