IBM 5250 was originally a family of terminal

Computer terminal

A computer terminal is an electronic or electromechanical hardware device that is used for entering data into, and displaying data from, a computer or a computing system...

 devices sold with the IBM

IBM

International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

 System/34

System/34

The IBM System/34 was a minicomputer marketed by IBM beginning in 1978. It was a multi-user, multi-tasking successor to the single-user System/32. Most notably, it included two very different processors, one based on System/32 and the second based on older System/3. Like the System/32 and the...

 minicomputer

Minicomputer

A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...

 systems. One model was the IBM 5251-11. It also connected to the later System/36, System/38 and AS/400

IBM System i

The IBM System i is IBM's previous generation of midrange computer systems for IBM i users, and was subsequently replaced by the IBM Power Systems in April 2008....

 systems.
Similar to the IBM 3270

IBM 3270

The IBM 3270 is a class of block oriented terminals made by IBM since 1972 normally used to communicate with IBM mainframes. As such, it was the successor to the IBM 2260 display terminal. Due to the text colour on the original models, these terminals are informally known as green screen terminals...

, it is a block-oriented terminal

Block-oriented terminal

A block-oriented terminal is a type of computer terminal that communicates with its host in blocks of data, usually chunks of text, as opposed to a character-oriented terminal that communicates with its host one character at a time...

 protocol, yet is incompatible with the 3270 standard. The relationship between the terminal and system was rich: The system sent a data stream containing markup bytes indicating the boundaries of data entry fields, highlighted or colored areas, and indication that the system was busy responding to the last request from the terminal.