Parrhesia

In rhetoric, parrhesia is a figure of speech described as: "to speak candidly or to ask forgiveness for so speaking". This Ancient Greek word has three different forms, as related by Michel Foucault. "Parrhesia" is a noun, meaning "free speech". "Parrhesiazomai" is a verb, meaning "to use parrhesia". "Parrhesiastes" is a noun, meaning one who uses parrhesia, for example "one who speaks the truth to power".
Etymology.
The term "parrhesia" is borrowed from the Greek παρρησία "parrhēsía" (πᾶν "all" and ῥῆσις "utterance, speech") meaning literally "to speak everything" and by extension "to speak freely", "to speak boldly", or "boldness". The term first appears in Greek literature, when used by Euripides, and may be found in ancient Greek texts from the end of the fifth century B.C. until the fifth century A.D. It implies not only freedom of speech, but the obligation to speak the truth for the common good, even at personal risk.
Usage in ancient Greece.
Parrhesia was a fundamental component of the democracy of Classical Athens. In assemblies and the courts Athenians were free to say almost anything, and in the theatre, playwrights such as Aristophanes made full use of the right to ridicule whomever they chose. Elsewhere there were limits to what might be said; freedom to discuss politics, morals, religion, or to criticize people would depend on context: by whom it was made, and when, and how, and where. 
If one was seen as immoral, or held views that went contrary to popular opinion, then there were great risks involved in making use of such an unbridled freedom of speech, as Socrates found out when he was sentenced to death for not adoring deities worshiped by the Athenians and for corrupting the young. 
Cynic philosophers.
Parrhesia was a central concept for the Cynic philosophers, as epitomized in the shameless speech of Diogenes of Sinope. 
Epicureans.
Parrhesia was also used by Epicureans in a friendly manner of frank criticism during teaching Epicurean philosophy and offering psychotherapy.
New Testament use.
A related use of parrhesia is found in the Greek New Testament, where it means "bold speech", the ability of believers to hold their own in discourse before political and religious authorities (e.g. Acts 4:13: "Now when they saw the boldness [την παρρησίαν] of Peter and John and realized that they were uneducated and ordinary men, they were amazed and recognized them as companions of Jesus."). It also is used to describe the reply Jesus made to the Pharisees.
Usage in rabbinic Jewish writings.
Parrhesia appears in Midrashic literature as a condition for the transmission of Torah. Connoting open and public communication, "parrhesia" appears in combination with the term δῆμος ("dimus", short for "dimosia"), translated "coram publica", in the public eye, i.e. open to the public. As a mode of communication it is repeatedly described in terms analogous to a commons. "Parrhesia" is closely associated with an ownerless wilderness of primary mytho-geographic import, the "Midbar Sinai" in which the Torah was initially received. The dissemination of Torah thus depends on its teachers cultivating a nature that is as open, ownerless, and sharing as that wilderness. The term is important to advocates of Open Source Judaism. Here is the text from the Mekhilta where the term "dimus parrhesia" appears (see bolded text).
Explanation: Why was the Torah not given in the land of Israel? In order that the peoples of the world should not have the excuse for saying: `Because it was given in Israel's land, therefore we have not accepted it.
The term "parrhesia" is also used in Modern Hebrew (usually spelled פַּרְהֶסְיָה), meaning "[in] public".
Modern scholarship.
Michel Foucault developed the concept of parrhesia as a mode of discourse in which one speaks openly and truthfully about one's opinions and ideas without the use of rhetoric, manipulation, or generalization. Foucault's use of parrhesia, he tells us, is troubled by our modern day Cartesian model of evidential necessity. For Descartes, truth is the same as the undeniable. Whatever can be doubted must be, and, thus, speech that is not examined or criticized does not necessarily have a valid relation to truth.
There are several conditions upon which the traditional Ancient Greek notion of parrhesia relies. One who uses parrhesia is only recognized as doing so if holding a credible relationship to the truth, if one serves as critic to either oneself or popular opinion or culture, if the revelation of this truth places one in a position of danger and one persists in speaking the truth, nevertheless, as one feels it is a moral, social, and/or political obligation. Further, in a public situation, a user of parrhesia must be in a social position less empowered than those to whom this truth is revealed.
Foucault (1983) sums up the Ancient Greek concept of parrhesia as such:
and
Foucault (1984) sums up that: