diff --git a/documentation/ref-manual/ref-variables.xml b/documentation/ref-manual/ref-variables.xml
index a72c01c614..214ef09f4a 100644
--- a/documentation/ref-manual/ref-variables.xml
+++ b/documentation/ref-manual/ref-variables.xml
@@ -928,6 +928,26 @@
             </glossdef>
         </glossentry>
 
+        <glossentry id='var-BUILD_OPTIMIZATION'><glossterm>BUILD_OPTIMIZATION</glossterm>
+            <glossdef>
+                <para>
+                    Specifies the optimization flags passed to the C compiler
+                    when building for the host or the SDK.
+                    The flags are passed through the
+                    <link linkend='var-BUILD_CFLAGS'><filename>BUILD_CFLAGS</filename></link>
+                    and
+                    <link linkend='var-BUILDSDK_CFLAGS'><filename>BUILDSDK_CFLAGS</filename></link>
+                    default values.
+                </para>
+
+                <para>
+                    The default value of the
+                    <filename>BUILD_OPTIMIZATION</filename> variable is
+                    "-O2 -pipe".
+                </para>
+            </glossdef>
+        </glossentry>
+
         <glossentry id='var-BUILDDIR'><glossterm>BUILDDIR</glossterm>
             <glossdef>
                 <para>