1 [computing] An instruction to a computer program, usually one word or concatenated words or letters, given by the user from a control device, such as a keyboard, or read from a file by a command interpreter.
2 [computing] A menu, menu item, button, combo box, text box, or tool on a toolbar.
3 [ESRI software] Any class in an ArcGIS system that implements the ICommand interface and can, therefore, be added to a menu or toolbar in an ArcGIS application.