Not surprisingly, it's @dict.
The supported shortcuts are:
@help - shows a list of shorcuts
@dict - opens dictionary to work after @dict; i.e. @dict zebra
@print - goes to kindle store
@store - goes to kindle store
@time - shows date and time
@url - open web browser to address after url; i.e. @url
www.mobileread.com
@web - open web browser to address after web; i.e. @web
www.mobileread.com
@wiki - open web browser to wikipedia
@wikipedia - open web browser to wikipedia