diff --git a/src/gui.h b/src/gui.h
index cca1ee3..b63125b 100644
--- a/src/gui.h
+++ b/src/gui.h
@@ -207,6 +207,8 @@
 #define INVALCOLOR (guicolor_T)-11111	/* number for invalid color; on 32 bit
 				   displays there is a tiny chance this is an
 				   actual color */
+#define CTERMCOLOR (guicolor_T)-11110	/* only used for cterm.bg_rgb and
+					   cterm.fg_rgb: use cterm color */
 
 #ifdef FEAT_GUI_GTK
   typedef PangoFontDescription	*GuiFont;       /* handle for a GUI font */