vivacity

Etymology

vivac(ious) + -ity, borrowed from Latin vīvācitās.

noun

1. The quality or state of being vivacious.
   But reposed natures may do well in youth. […] On the other side, heat and vivacity in age, is an excellent composition for business. 1612, Francis Bacon, “Of Youth and Age”, in Essays or Counsels, Civil and Moral
   1738, David Hume, A Treatise of Human Nature, Book I, Part I, Section III. Of the Ideas of the Memory and the Imagination, We find by experience, that when any impression has been present with the mind, it again makes its appearance there as an idea; and this it may do after two different ways: either when in its new appearance it retains a considerable degree of its first vivacity, and is somewhat intermediate betwixt an impression and an idea: or when it entirely loses that vivacity, and is a perfect idea.
   The one entertained me with her vivacity when I was gay, the other with her sense when I was serious. 1766, Oliver Goldsmith, chapter 1, in The Vicar of Wakefield
   Some secret sorrow, or the brooding spirit of some moody passion, had quenched the light and ingenuous vivacity of youth in a countenance singularly fitted to display both […] 1819, Walter Scott, chapter 5, in The Bride of Lammermoor
   […] an extraordinary observer might have seen that the chin was very pointed and pronounced; that the big eyes were full of spirit and vivacity; that the mouth was sweet-lipped and expressive; that the forehead was broad and full; in short, our discerning extraordinary observer might have concluded that no commonplace soul inhabited the body of this stray woman-child […] 1908, Lucy Maud Montgomery, chapter 2, in Anne of Green Gables