Nicknames are aliases for full map names. To obtain a list of available map nicknames, such as passwd for passwd.byname, type ypcat -x or ypwhich -x.
Nicknames are stored in the /var/yp/nicknames file, which contains a map nickname followed by the fully specified name for the map, separated by a space. This list might be added to or modified. Currently, there is a limit of 500 nicknames.