It's not a unit of measure. It's a kind of thing. In a recipe, lemon zest or orange zest is the thin outermost part of the rind. When grated into fine bits, it is used as flavoring.
In cooking, the zest is the thin outer peel of a citrus fruit, obtained by grating the peel of the fruit until one sees the white of the fruit. In a lemon, it's about a tablespoon. In an orange, it's about 3 tablespoon.
I envied their zest for life
There was a pest in the zest! -poop
The chef added some zest to the food with the spices.
Chi?
Zeal.
The word is gusto!
Zest could be used to describe a quality person who is filled with enthusiasm and energy.
zany, zest
brio, zeal, zest
zinc, zest, zeal
zest
Xois Yuia Zest