diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
index 85dcc041d089..27efb72b62e0 100644
--- a/lib/matplotlib/axes/_axes.py
+++ b/lib/matplotlib/axes/_axes.py
@@ -5129,6 +5129,16 @@ def imshow(self, X, cmap=None, norm=None, aspect=None,
         coordinates. In other words: the origin will coincide with the center
         of pixel (0, 0).
 
+        Two typical representations are used for RGB images with an alpha
+        channel:
+
+        -   Straight (unassociated) alpha: R, G, and B channels represent the
+            color of the pixel, disregarding its opacity.
+        -   Premultiplied (associated) alpha: R, G, and B channels represent
+            the color of the pixel, adjusted for its opacity by multiplication.
+
+        `~matplotlib.pyplot.imshow` expects RGB images adopting the straight
+        (unassociated) alpha representation.
         """
 
         if not self._hold: