Adherence to the highest principles and ideals.
Probity and its synonyms honesty, honor, and integrity all mean uprightness of character or action, with some slight differences in emphasis. Honesty implies a refusal to lie or deceive in any way. Honor suggests an active or anxious regard for the standards of one's profession, calling, or position. Integrity implies trustworthiness and incorruptibility to a degree that one is incapable of being false to a trust, responsibility, or pledge. Probity, which descends from Latin probus, meaning "honest," implies tried and proven honesty or integrity.
From Wiktionary:
Integrity, especially of the quality of having strong moral principles; honesty and decency. From Middle French probité, from Latin probitas (“uprightness, honesty”), from probus (“good, excellent, honest”).