Clarify "cardinality of" as "number of elements in" as many readers do
not have a math vocabulary.
diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
index 774b0c6..e491fd2 100644
--- a/Doc/library/stdtypes.rst
+++ b/Doc/library/stdtypes.rst
@@ -3828,7 +3828,7 @@
 
    .. describe:: len(s)
 
-      Return the cardinality of set *s*.
+      Return the number of elements in set *s* (cardinality of *s*).
 
    .. describe:: x in s