Viewing angle refers to a range of angles where the image displayed on a monitor remains suitable to the user. Generally, all monitors will show an optimum image when you're sitting near the center of the panel. However, things can quickly go awry when you shift the position of your head left, right, up or down depending on what type of panel your monitor users.
Regarding vertical viewing angles, it's generally accepted that the center of the display should be somewhere between eye level and thirty degrees below your line of sight. Anything outside those parameters for vertical viewing angles could lead to eye strain. However, every person is different, and depending on your height, desk height and other factors, it might not be possible to hit this "sweet spot" in practice.
With LCD technology used in today's computer monitors, the horizontal viewing angle typically maxes out at 178 degrees. But almost no one views a monitor at such extreme angles, and monitor manufacturers often fudge the numbers a bit when it comes to this specification. So, for example, two monitors could both be rated with a 178-degree horizontal viewing angle yet exhibit widely different performance which reaching the outer limits of that range.
So, what happens when you reach the edge of the viewing angle cone of a monitor? Depending on the monitor in question, you may see various amounts of brightness drop-off, a decrease in contrast ratio and color shifting. As you can see in the image above, IPS (in-plane switching) panels tend to have less color shifting off-axis than VA (vertical alignment) panels. TN (twisted nematic) panels, a much older technology, tend to be far interior to IPS and VA in both horizontal and vertical viewing angles.
Monitor manufacturers have attempted to compensate for some of these viewing angle deficiencies inherent with curved VA panels, which are popular in the gaming space.
This article is part of the Tom's Hardware Glossary.