What is Zest?

Zest definition and meaning on Dictionary terms:

keen relish; hearty enjoyment; gusto.
an agreeable or piquant flavor imparted to something.
anything added to impart flavor, enhance one’s appreciation, etc.
piquancy; interest; charm.
liveliness or energy; animating spirit.
the peel, especially the thin outer peel, of a citrus fruit used for flavoring: lemon zest.