diff --git a/LEGO1/realtime/roi.h b/LEGO1/realtime/roi.h
index 0f2cf985..6f2d1339 100644
--- a/LEGO1/realtime/roi.h
+++ b/LEGO1/realtime/roi.h
@@ -1,6 +1,8 @@
 #ifndef ROI_H
 #define ROI_H
 
+// ROI stands for Region of Interest.
+
 #include "../compat.h"
 #include "../mxstl.h"
 #include "../realtime/realtime.h"