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Censorship means intentional effort towards preventing exchange of certain kind of information among other individuals, for example suppression of free speech, altering old works of art for political reasons, forced takedowns of copyrighted material from the Internet etc. Censorship is opposing free speech. Note that thereby censorship does NOT include some kinds of data or information filtering, for example censorship does not include filtering out noise such as spam on a forum or static from audio (as noise is a non-information) or PERSONAL avoidance of certain information (e.g. using adblock or hiding someone's forum posts ONLY FOR ONESELF). Censorship is always wrong -- in a good society there is never a slightest reason to censor anything, therefore whenever censorship is deemed the best solution, something within the society is deeply fucked up. In current society censorship, along with propaganda, brainwashing and misinformation, is extremely prevalent and growing -- it's being pushed not only by governments and corporations but also by harmful terrorist groups such as LGBT and feminism who force media censorship (e.g. that of Wikipedia or search engines) and punishment of free speech (see political correctness and "hate speech").
Sometimes it is not 100% clear which action constitutes censorship: for example categorization such as moving a forum post from one thread to another (possibly less visible) thread may or may not be deemed censorship -- this depends on the intended result of such action; moving a post somewhere else doesn't remove it completely but can make it less visible. Whether something is censorship always depends on the answer to the question: "does the action prevent others from information sharing?".
There exist tools for bypassing censorship, e.g. proxies or encrypted and/or distributed, censorship-resistant networks such as Tor, Freenet, I2P or torrent file sharing. Watch out: using such tools may be illegal or at least make you look suspicious and be targeted harder by state surveillance.
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