A condition which affects laserdiscs caused by the deterioration of the aluminum layer of the disc due to oxidation. This deterioration causes the disc to develop spots and speckles all over it that change the reflectivity, ultimately making the afflicted media impossible to view (the audio will cut out, the video will skip or both).
Contrary to the name, laser rot is not caused by laser light. The large size of laserdiscs makes them flexible. Since laserdiscs are created by gluing together two discs flexing along the bond between the discs can break the seal, allowing air and moisture to come in contact with the delicate aluminum layer. The glue used to attach both sides of a laserdisc can also eventually eat away at the recorded material. This process can occur very rapidly with improperly manufactured discs. When laserdiscs were still being produced certain factories were infamous for churning out endless runs of rot-prone discs -- to the point where sending any disc from these factories in for a replacement due to laser rot was futile as the replacement would rot just as quickly.
DVDs are much smaller and more rigid than laserdiscs. Also DVDs are molded from polycarbonate which absorbs about 10 times less moisture than the polymethyl methacrylate used for laserdiscs. These properties make DVDs highly resistant to laser rot (although they may still be susceptable to the fungus that eats compact discs).