This definition of Censorship seems to reflect my own viewpoint on the subject:
"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."
http://www.pbs.org/wgbh/cultureshock...finitions.html
According to another site, the quote comes from Franklin S. Haiman:
http://msupress.msu.edu/authorbio.php?authorID=1716
Unfortunately, I can't seem to find any documents which confirm that Haiman actually said it. Another site claims that the quote came from a transcript of of "The Censorship Conference of the National Ad Hoc Committee Against Censorship" which took place in New York City in 1975. However, I can't seem to find any information regarding this conference.
So, long story short, while I can't confirm the origin of this quote, it seems to me to be a reasonable definition of censorship.