Siddiq

Siddiq ( ; from the Arabic word meaning "truthful") is an Islamic term and is given as an honorific title to certain individuals. The feminine gender for Siddiq is Siddiqah. The word is sometimes used as a title given to individuals by Muhammad, according to Islamic sources. Otherwise, it is used to denote that the person is totally trustworthy.
Sunni usage.
Sunni Muslims use as-Siddiq as an epithet for Abu Bakr, the first Caliph of Islam and the closest friend of Muhammad, while they use Siddiqah for Aisha, Abu Bakr's daughter and the Prophet's wife.
Sufi term.
In Sufism, Siddiq is a rank that comes after prophet. It is generally given to a person who verified the claim of prophethood in its early stage. Sufis believe the following four ranks are free of time and space and therefore life and death becomes meaningless to them. 
These four ranks are mentioned in the Quran.
Shia usage.
Shi'a Muslims use Siddiqah as a title for Muhammed's daughter Fatima. In "Then I was Guided", the Shi'a author asks its Sunni audience how it is possible for both Abu Bakr and Fatimah to be "Siddiq", when contemplating their intense and deep conflicts, implying that logic demands one to be a liar, and arguing it can not be Muhammed's daughter. The Sunni view is that Muhammad daughter Fatimah is sidiqqa.
Other usage.
In Hebrew the word/name "Tzadik" (צדיק), has a similar meaning. The title of Voltaire's satirical novella "Zadig" also stems from this name root.