With regard to audio and video, Aliasing is a distortion (an artifact) that happens when sampling a signal under some conditions. The term is used to describe several occurrences that cover the field of computing in general. A good example of Aliasing with regard to video is temporal aliasing when the frequency is too low and we get the wagon-wheel effect. This would cause a spoked wheel to look like its rotating differently to what it actually was when it was recorded. The illusion is that the wheel is going slower or backwards.
This is easier to imagine if you think of significantly reducing the frame rate of a 30fps video, in which a spoked wheel appears to be rotating properly. However, when the frame rate change occurs, the less images (less samples is better term) of the wheel rotating may make it look like it's rotating slower, or if the wheel is moving fast in the original record, it may look backwards.
Aliasing effects may also have an effect of a very "busy" area of video footage. For example, a television presenter may wear a heavily striped t-shirt while reporting the news. However, through the processing that occurs, when you see it on your TV screen, there may be additional noise produced, distorting the t-shirt.