truthfulness

noun

1. Freedom from deceit or falseness: truth, veracity. See true/false.
2. The quality of being authentic: authenticity, genuineness, realness, validity. See true/false.

The virtue of telling the truth. This can be elevated into the most stringent duty, as in the philosophy of Kant, where it extends to the duty to tell the mad axeman where your children are sleeping, if there is no way of ducking the question. A less demanding morality might balance the duty against other obligations, or recognize only a duty to tell the truth to those who have a right to the truth.

The noun has one meaning:

Meaning #1: the quality of being truthful
  Antonym: untruthfulness (meaning #1)