window.open() command. APEX does this for Popup LOV items, the legacy datepicker, and within Application Builder for online help, to name a few examples.
The challenging issue with popup windows is you can't make them modal in any standard way across the major browsers. The second issue with popup windows is that you cannot completely hide the browser "chrome", or border components of a web browser window, which includes window frames, menus, toolbars, and scroll bars.
The modal Ext.Window with an embedded iFrame page, shown ...