folding 

in spectroscopy
Synonym: aliasing
https://doi.org/10.1351/goldbook.08248
The appearance of peaks at incorrect frequencies in a mathematically-processed spectrum due to insufficient density of datapoints in the time-domain spectrum or from peaks outside of the sampled frequency range.This process can be simulated by folding a spectrum that shows the correct wavenumbers back onto itself at ν~M and its multiples.
Notes:
  1. Folding is a consequence of undersampling in spectroscopy.
  2. In vibrational spectroscopy, when a spectrum is sampled correctly only up to a wavenumber ν~M, radiation of wavenumber ν~>ν~M appears to have a wavenumber between 0 and ν~M, per the table below:
Range of wavenumber ν~ν~app
[ν~M, 2ν~M]2ν~M - ν~
[2ν~M, 3ν~M]ν~ - 2ν~M
[3ν~M, 4ν~M]4ν~M - ν~
[4ν~M, 5ν~M]ν~ - 4ν~M
Source:
PAC, 2021, 93, 647. (Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)) on page 659 [Terms] [Paper]