The IBM 3270 is a class of block oriented terminals made by IBM originally introduced in 1972 (known as "display devices") normally used to communicate with IBM mainframes. The 3270 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. Unlike common serial ASCII terminals, the 3270 minimizes the number of I/O interrupts required by transferring large blocks of data known as data streams, and uses a high speed proprietary communications interface, using coaxial cable.
Although IBM no longer manufactures 3270 terminals the IBM 3270 protocol is still commonly used via terminal emulation to access some mainframe-based applications. Accordingly, such applications are sometimes referred to as green screen applications. Use of 3270 is slowly diminishing over time as more and more mainframe applications acquire Web interfaces, but some web applications use the technique of "screen scraping" to capture old screens and transfer the data to modern front-ends.
Read more about IBM 3270: Principles, Third Parties, Models, Manufacture, Telnet 3270