Piety, n.
Etymology: [F. piété; cf. It. pietà; both fr. L. pietas piety, fr. pius pious. See Pious, and cf. Pity.]
1. Veneration or reverence of the Supreme Being, and love of his character; loving obedience to the will of God, and earnest devotion to his service. Piety is the only proper and adequate relief of decaying man. Rambler.
2. Duty; dutifulness; filial reverence and devotion; affectionate reverence and service shown toward parents, relatives, benefactors, country, etc. Conferred upon me for the piety Which to my country I was judged to have shown. Milton.
Syn. -- Religion; sanctity; devotion; godliness; holiness. See Religion.
Shades of meaning
Fernald has no entry of its own for piety; he lists it under religion.
Where he writes about it:
Piety is primarily filial duty, as of children to parents, and hence, in its highest sense, a loving obedience and service to God as the Heavenly Father; pietism often denotes a mystical, sometimes an affected piety; religion is the reverent acknowledgment both in heart and in act of a divine being. — from religion