diff --git a/doc/guide/preamble.sdf b/doc/guide/preamble.sdf
index 71a694fb6f..30b4b451cc 100644
--- a/doc/guide/preamble.sdf
+++ b/doc/guide/preamble.sdf
@@ -187,6 +187,7 @@ URI|Uniform Resource Identifier
 URL|Uniform Resource Locator
 WWW|World Wide Web
 X.500|X.500 Directory Services
+X.509|X.509 Public Key and Attribute Certificate Frameworks
 !endblock
 
 !block references; data