diff --git a/book/source/glossary.md b/book/source/glossary.md
index 7ba86b8..5994e9e 100644
--- a/book/source/glossary.md
+++ b/book/source/glossary.md
@@ -183,7 +183,7 @@ Expiration Time
     The time of expiry of an {term}`OpenPGP Signature Packet`.
 
 Features Subpacket
-    A {term}`OpenPGP Signature Subpacket`, which denotes advanced OpenPGP features an {term}`implementation<OpenPGP Implementation>` supports.
+    An {term}`OpenPGP Signature Subpacket`, which denotes advanced OpenPGP features an {term}`implementation<OpenPGP Implementation>` supports.
 
     For an in-depth view on these {term}`subpackets<OpenPGP Signature Subpacket>` see [](zoom-dks).