Description of the practice of using speech that conforms to liberal or radical opinion by avoiding language which might cause offence to or disadvantage social minorities.
Please note: it is difficult to discuss the meaning and origin of the term 'politically correct' whilst avoiding expressing political opinion. I have attempted to do that below.
The terms 'politically correct' and 'political correctness', in the sense defined above, entered the language via the U.S. feminist and other left-wing movements of the 1970s. The use of 'PC' language quickly spread to other parts of the industrialized world. The terms had been used previously though. The previous meaning was 'in line with prevailing political thought or policy'. I.e. the terms previously used 'correctness' in its literal sense and without any particular reference to language that some might consider illiberal or discriminatory. That usage dates back to the 18th century. For example, J. Wilson's comments in U.S. Republic, 1793:
"The states, rather than the people, for whose sake the states exist, are frequently the objects which attract and arrest our principal attention... Sentiments and expressions of this inaccurate kind prevail in our common, even in our convivial, language... ‘The United States’, instead of the ‘People of the United States’, is the toast given. This is not politically correct."
The earliest printed reference that is unambiguous in it's use of 'politically correct' in its current commonly understood sense is Toni Cade's The Black Woman, 1970:
"A man cannot be politically correct and a chauvinist too."
I should add that the use of, or even the definition of, 'political correctness' as seen by the liberal left is strongly disputed by those of other political views (and even by many liberals). Some view the very term 'politically correct' to be pejorative in that it portrays a political stance that they oppose as 'correct'.
There are strong views on both sides. It isn't difficult to find examples of moves to modify language that are clearly misguided. For example, the attempt by some in the UK to discourage the use of the term 'nitty-gritty', which was mistakenly thought to be disparaging to black people. On the other side of the coin there are many examples of gender biased language - e.g. chairman used when the person chairing a meeting is female - that are linguistically incorrect (although some would dispute that too). This topic of gender neutrality is possibly the area that is most contentious. Some would argue that any use of the word 'man', e.g. manhole, is biased and should be avoided. Others are quite happy with female chairmen.
The extreme polarity of views on this topic is encapsulated in the story that radical feminists lobbied the UK government to have Manchester renamed as Personchester. This is a myth and those that support the use of PC language point to it as an example of the right-wing press attempting to discredit their views by spreading false rumours.