Baseline correction for continuous EEG signals
A baseline is generally an unwanted artifact that alter interpretation or computation on signals. The concept is not totally well-defined, and several different names coexist, with similar meaning:  drift, continuum, trend, background. 

It is often described as a large-scale, slowly-varying reference level, that should be removed to compare, or adjust the signal with respect to a globally flat or  zero-reference. It can be seen as a generalized (non-constant) offset or bias  correction. When removed, it is generally simpler to further process the signal, quantify parameters, extract features.

In some domains, this smooth reference may also  include random noise. 

Similar problems are answered here:


How to remove or filter the drift problem in measured Strain signal?
Help with automated baseline removal