A Detrending Method in Data Processing
We all know that spectral estimation request that the sigal is at least weakly stationary. So, detrending is just trying to remove the slow nonstationary trends from the data that need processing.
This is a detrending method using smoothness priors aproach(SPA).
Ref. An advanced detrending method with application to HRV analysis. IEEE trans. on Biomedical Engineering
An example for detrending. the sinusoidal signal that have lower frequency is used as the trend.