An expression is the communication of one's beliefs or opinions through free speech, writing and other methods such as music and other arts. Some people may find other people's expressions offensive, even defamatory, and some expressions can be considered libel.
In some countries, freedoms taken for granted elsewhere, are commonly oppressed by the government. At the moment the Chinese communist party strictly imposes a ban on any speech which is opposed to or critical of itself.
In programming, an expression is any permissable combination of symbols that represents a value.