When configuring the Edit Tool widget in the Web AppBuilder, there is an option to “enable” the Merge tool. If enabled, it appears to the user in the Edit dialog window as the Union tool. From an ArcMap perspective, Merge seems to be the more appropriate name, given its functionality (see link,below). Either way, there should be consistency between these two dialog windows.
http://resources.arcgis.com/en/help/main/10.2/index.html#//01m700000062000000
