Aliasing error: If the signal x(t) is not strictly band limited and or if sampling frequency $f_s$ is less than twice. The maximum frequency w, i.e. $f_s$ < 2w, then the error called aliasing or fold over error occurs. In short if sampling the cozen is not satisfied error occurs.

This distortion occurs due to over lapping of adjacent spectrums. Due to this overlapping of adjacent spectrums. Due to this overlapping portions of frequency shifted replicas are folded over desired spectrum. That is this phenomenon of a high frequency in the spectrum of the original signal x(t), taking on the identity of lower frequency in the spectrum of sampled signal x (t) is called aliasing or fold over error.

Aliasing error can be eliminated by using

1. Antialising or prealicsing filter
2. Keeping sampling frequency greater than twice the maximum frequency i.e. $f_s$ > 2w
3. Ant aliasing is a band limiting low pass filter (LPF) and pass x (t) through it before sampling. This filter has cut off frequency at $f_c$ = w so it will strictly bandlimit signal x (t) before sampling.