According to Merriam Webster Dictionary, Censor simply means a person who supervises conducts or morals. A hypothetical psychic agency that represses unacceptable notions before they reach consciousness. If so, then censorship simply means the act of suppressing/condenming of contents that are deemed subversive of the common good. From the above clarifications, you would see censorship as a helpful instrument in maintaining the moral behavior in the community. However, it's usefulness is basically dependent on the person/agrncy (censor) determining the (un)acceptability of such notions/contents. The censorship/prohibition of certain drugs is one of the effective ways of preventing the accessibility of such drugs by the abusers. Also the censorship of some contents (movies specifically) reduces the chances of the contents getting to the wrong audience.
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The concept of censorship occurs in all manifestation of authority but in our current world, it has been of special importance in relative to the government and the rule of law. In this regard, I call it the second face of the two-faced knife (censorship). The best way of avoiding critics as a government is by having control over what the citizens say or hear. This is well practiced in my country where the media literally works in favour of the government and speaking up against any actions of the government attracts its own punishment, as you would be called up for questioning by the EFCC or DSS.
The instances above clearly illustrates what I mean by Censorship — the two-faced knife. It's employment by some agencies to curb some harmful actions or bad conducts in order to promote good morals amongst the people is the positive side of the knife. While it's employment by government agencies to control its citizens — modern slavery, is the other face of the knife (the negative side).