"Jaggies" is the informal name for artifacts in raster images, most frequently from aliasing, which in turn is often caused by non-linear mixing effects producing high-frequency components or missing or poor anti-aliasing filtering prior to sampling.
Jaggies are stairlike lines that appear where there should be smooth straight lines or curves. For example, when a nominally straight, un-aliased line steps across one pixel, a dogleg occurs halfway through the line, where it crosses the threshold from one pixel to the other.
Jaggies should not be confused with most compression artifacts, which are a different phenomenon.
Jaggies can occur for a variety of reasons, the most common being that the output device (display monitor or printer) does not have enough resolution to portray a smooth line. In addition, jaggies often occur when a bit-mapped image is converted to a different resolution. This is one of the advantages that vector graphics has over bit-mapped graphics – the output looks the same regardless of the resolution of the output device.
The effect of jaggies can be reduced somewhat by a graphics technique known as spatial anti-aliasing. Anti-aliasing smooths out jagged lines by surrounding the jaggies with transparent pixels to simulate the appearance of fractionally-filled pixels. The downside of anti-aliasing is that it reduces contrast – rather than sharp black/white transitions, there are shades of gray – and the resulting image is fuzzy. This is an inescapable trade-off: if the resolution is insufficient to display the desired detail, the output will either be jagged or fuzzy, or some combination thereof.
In realtime computer graphics, especially gaming, anti-aliasing is used to remove jaggies created by the edges of polygons and other lines entirely. Some newer game consoles, such as the Xbox 360 and PlayStation 3, had publishing policies which mandated the use of anti-aliasing in all games released for them. Jaggies in bitmaps, such as sprites and surface materials, are most often dealt with by separate texture filtering routines, which are far easier to perform than anti-aliasing filtering. Texture filtering became ubiquitous on PCs after the introduction of 3Dfx's Voodoo GPU, and on game consoles following the Nintendo 64's debut.
Notable uses of the term 
In the Atari 8-bit game Rescue on Fractalus!, published by Lucasfilm Games in 1985, the graphics depicting the cockpit of the player's spacecraft contains two window struts, which are not anti-aliased and are therefore very "jagged". The developers made fun of this and named the in-game enemies "Jaggi", and also initially titled the game Behind Jaggi Lines!. The latter idea was scrapped by the marketing department before release.
- Mitchell, Don, "The Antialiasing Problem in Ray Tracing", Advanced Topics in Ray Tracing, Course Notes, SIGGRAPH 90.
- Interview with David Fox (from: James Hague: Halcyon Days: Interviews with Classic Computer and Video Game Programmers)