diff --git a/doc/manual/source/the-model-file.rst b/doc/manual/source/the-model-file.rst
index d2fc0c6a4c5d4a2a5cdeccd5bae1dfe66023e6dc..edf8c5e53da2148281d024541978c4d50780ac00 100644
--- a/doc/manual/source/the-model-file.rst
+++ b/doc/manual/source/the-model-file.rst
@@ -615,6 +615,10 @@ MODEL_EXPRESSION and EXPRESSION:
 
     Square root.
 
+.. function:: cbrt(x)
+
+    Cubic root.
+
 .. function:: sign(x)
 
     Signum function, defined as: