diff --git a/posix-compliance/glossary.rst b/posix-compliance/glossary.rst
index 811d338..5944150 100644
--- a/posix-compliance/glossary.rst
+++ b/posix-compliance/glossary.rst
@@ -5,3 +5,8 @@ Glossary
 
 .. glossary::
    :sorted:
+
+   POSIX
+      Portable Operating System Interface is a family of standards specified
+      by the IEEE Computer Society for maintaining compatibility between
+      operating systems.