Used most often among laserdisc collectors, "laser rot" describes a progressive condition that causes the video and audio of a laserdisc to become distorted and/or unplayable. The deterioration of audio/video is usually attributable to manufacturing defects (though it has also been attributed to improper storage by users), and tends to appear most frequently in specific batches by specific manufacturers. Early MCA DiscoVision titles and mid/late Sony USA releases are most prone to laser rot. For the most part, no matter the reason for the "rot"- inclusion defects in the glue, exposure to the elements, impure substrates- the deterioration of the audio/video is due to corrosion of the aluminum reflective layer, and LD players using shorter-wavelength lasers can often read through "rot" that stymies other players.
Rot on laserdiscs usually manifests itself as multi-colored "snow"/speckles in the picture frame, and tends to be worse at the beginning and end of a LD side, corresponding to edges of the LD itself. Once laser rot sets in on a LD, it worsens over time, often rendering the LD unplayable or unwatchable. Typical wisdom holds that laser rot manifests itself within a couple of years of manufacture, though some cases of laser rot beginning after longer periods of time are known.
Similar conditions may affect other optical disc formats, and some people use "laser rot" as a catch-all term to describe failure-over-time of CDs and DVDs, even if the reasons for those failures may not be due to the same defects that caused rot in laserdiscs.
When I first watched my copy of Witchcraft VII, it was fine... and when I popped it in last week, I couldn't watch the sex scenes through all of the snow... damn laser rot.