One would think it would be an easy definition, after all, we know what censorship is, right?
Censor: One who supervises conduct and morals: as a) an official who examines materials (as publications or films) for objectionable matter; b) an official (as in time of war) who reads communications (as letters) and deletes material considered harmful to the interests of his organization. Censorship: The institution, system or practice of censoring; the actions or practices of censors; esp : censorial control exercised repressively.
--Merriam Webster's Collegiate Dictionary
In general, censorship of books is a supervision of the press in order to prevent any abuse of it. In this sense, every lawful authority, whose duty it is to protect its subjects from the ravages of a pernicious press, has the right of exercising censorship of books.
--The Catholic Encyclopedia (a publication of the Catholic Church)
--The American Library Association
Censorship is a word of many meanings. In its broadest sense it refers to suppression of information, ideas, or artistic expression by anyone, whether government officials, church authorities, private pressure groups, or speakers, writers, and artists themselves. It may take place at any point in time, whether before an utterance occurs, prior to its widespread circulation, or by punishment of communicators after dissemination of their messages, so as to deter others from like expression. In its narrower, more legalistic sense, censorship means only the prevention by official government action of the circulation of messages already produced. Thus writers who "censor" themselves before putting words on paper, for fear of failing to sell their work, are not engaging in censorship in this narrower sense, nor are those who boycott sponsors of disliked television shows.
--Academic American Encyclopedia
Censorship: supervision and control of the information and ideas circulated within a society. In modern times, censorship refers to the examination of media including books, periodicals, plays, motion pictures, and television and radio programs for the purpose of altering or suppressing parts thought to be offensive. The offensive material may be considered immoral or obscene, heretical or blasphemous, seditious or treasonable, or injurious to the national security.