Definition - What does Eye Diagram mean?
An eye diagram is an indicator of quality in a serial bit stream. The display (which has symmetrical patterns shaped similarly to eyes, hence the name) represents the integrity and consistency of data streams by visualizing the transitions between 0 and 1 values and comparing those transitions among the others in their stream.
An eye diagram is also known as an eye pattern.
Techopedia explains Eye Diagram
Eye diagrams are displayed using oscilloscopes, which overlay segments of data streams to produce graphic representations that look similar to open eyes. If the edges of these shapes appear solid and consistent (or "stable") then the signal is good and has integrity. A bad signal, conversely, shows inconsistent lines and less-defined shapes.
As data streams increase in rate, volume and complexity, the demand for consistency and integrity makes eye patterns highly useful for engineers.