The word "utopia," in its most common meaning, refers to a socially or politically perfect state or nation. Alternatively, it can also be understood to mean the human efforts to create a better society, or more specifically, a perfect society that does not yet exist.
Ideas which could radically change societies or governments are may be referred to as "utopian ideas," often in a negative tone. Similarly, "utopia" can be used in a negative way; often to discredit ideas deemed too optimistic, unrealistic, or hopeful to be practically realized in the world as it exists today.
"Utopia" has also been used to describe actual communities founded in an attempt to create a perfect or utopian society.
Conversely, the word "dystopia," presumably a derivative of the word "utopia," can be used to refer to a failed Utopia; that is, to a nightmarish civilization with laws or a social order contrary to the utopian ideal.