Docs for CTR and GCM should say that IV/nonce must be unique
diff --git a/docs/glossary.rst b/docs/glossary.rst
index dc6f3eb..202fa2d 100644
--- a/docs/glossary.rst
+++ b/docs/glossary.rst
@@ -64,3 +64,11 @@
text
This type corresponds to ``unicode`` on Python 2 and ``str`` on Python
3. This is equivalent to ``six.text_type``.
+
+ nonce
+ A nonce is a **n**\ umber used **once**. Nonces are used in many
+ cryptographic protocols. Generally, a nonce does not have to be secret
+ or unpredictable, but it must be unique. A nonce is often a random
+ or pseudo-random number (see :doc:`Random number generation
+ </random-numbers>`). Since a nonce does not have to be unpredictable,
+ it can also take a form of a counter.