Something other that what it appears to be. Stairsteps on what should be a smooth diagonal line are an example of spatial alias.

Wagon wheels appearing to move backwards are an example of temporal alias. Aliases are caused by sampling and can be reduced or eliminated by pre-filtering, which can appear to be a blurring effect. Defects in the picture typically caused by insufficient sampling (violation of the Nyquist sampling rate) in the analog-to-digital conversion process or poor filtering of digital video. Defects are typically seen as jaggies on diagonal lines and twinkling or brightening in picture detail. Examples are: Temporal Aliasing – such as rotating wagon wheel spokes appearing to rotate in the reverse direction. Raster Scan Aliasing – such as sparkling or pulsing effects in sharp horizontal lines. Stair-Stepping – stepped or jagged edges in diagonal lines or the diagonal parts of a letter.