Nosism: The use of "we" in referring to oneself. From Latin nos, meaning "we." Also known as the royal we ("We are not amused"--Queen Victoria) or the editorial we ("We recommend a No vote on Proposition 785"--the Messenger-Tribune).
"Only kings, presidents, editors, and people with tapeworms have the right to use the editorial 'we'." -- Mark Twain.
Hat tip to Anu Garg of Wordsmith.