Given a power spectrum (a plot of power vs. frequency), aliasing is a false translation of power falling in some frequency range outside the range. Aliasing can be caused by discrete sampling below the Nyquist Frequency. The sidelobes of any Instrument Function (including the simple Sinc Squared function obtained simply from Finite sampling) are also a form of aliasing. Although sidelobe contribution at large offsets can be minimized with the use of an Apodization Function, the tradeoff is a widening of the response (i.e., a lowering of the resolution).
See also Apodization Function, Nyquist Frequency