MRG, DOC: Fix ICA docstring, add whitening

doc/glossary.rst

@@ -286,6 +286,17 @@ general neuroimaging concepts. If you think a term is missing, please consider
         A coordinate frame affine transformation, usually between the Neuromag head
         coordinate frame and the MRI Surface RAS coordinate frame used by Freesurfer.
 
+    whitening
+        A linear operation that transforms data with a known covariance
+        structure into "whitened data" which has a covariance structure that
+        is the identity matrix (i.e., it creates virtual channels that are
+        uncorrelated and have unit variance).
+
+        The term "whitening" comes from the fact that light with a flat
+        frequency spectrum in the visible range is white, whereas
+        non-uniform frequency spectra lead to perception of different colors
+        (e.g., "pink noise" has a ``1/f`` characteristic, which for visible
+        light would appear pink).
 
 .. LINKS
 

mne/preprocessing/ica.py

@@ -438,7 +438,7 @@ def fit(self, inst, picks=None, start=None, stop=None, decim=None,
 
         Parameters
         ----------
-        inst : instance of Raw, Epochs or Evoked
+        inst : instance of Raw or Epochs
             Raw measurements to be decomposed.
         %(picks_good_data_noref)s
             This selection remains throughout the initialized ICA solution.

tutorials/evoked/plot_whitened.py

@@ -2,7 +2,8 @@
 Plotting whitened data
 ======================
 
-This tutorial demonstrates how to plot whitened evoked data.
+This tutorial demonstrates how to plot :term:`whitened <whitening>` evoked
+data.
 
 Data are whitened for many processes, including dipole fitting, source
 localization and some decoding algorithms. Viewing whitened data thus gives