Definition of "æra"

It is desirable that all people should adopt one and the same æra, independant of moral revolutions, and founded solely on astronomical phænomena.

1798, Pierre Simon La Place, “Exposition du Système du Monde [On the System of the World]”, in The Monthly Review, new and improved‎, page 500

In the fourth century of our æra we find this word made plural, and even feminine, and used as the equivalent of Parcæ.

1892, Thomas Keightley, Fairy Mythology, London: George Bell and Sons, page 5