The Collaborative International Dictionary of English v.0.48:

Zest \Zest\ (z[e^]st), n. [F. zeste, probably fr. L. schistos
   split, cleft, divided, Gr. ?, from ? to split, cleave. Cf.
   Schism.]
   [1913 Webster]
   1. A piece of orange or lemon peel, or the aromatic oil which
      may be squeezed from such peel, used to give flavor to
      liquor, etc.
      [1913 Webster]

   2. Hence, something that gives or enhances a pleasant taste,
      or the taste itself; an appetizer; also, keen enjoyment;
      relish; gusto.
      [1913 Webster]

            Almighty Vanity! to thee they owe
            Their zest of pleasure, and their balm of woe.
                                                  --Young.
      [1913 Webster]

            Liberality of disposition and conduct gives the
            highest zest and relish to social intercourse.
                                                  --Gogan.
      [1913 Webster]

   3. The woody, thick skin inclosing the kernel of a walnut.
      [Obs.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Zest \Zest\, v. t. [imp. & p. p. Zested; p. pr. & vb. n.
   Zesting.]
   1. To cut into thin slips, as the peel of an orange, lemon,
      etc.; to squeeze, as peel, over the surface of anything.
      [1913 Webster]

   2. To give a relish or flavor to; to heighten the taste or
      relish of; as, to zest wine. --Gibber.
      [1913 Webster]

WordNet (r) 3.0 (2006):

zest
    n 1: vigorous and enthusiastic enjoyment [syn: gusto,
         relish, zest, zestfulness]
    2: a tart spicy quality [syn: nip, piquance, piquancy,
       piquantness, tang, tanginess, zest]
    v 1: add herbs or spices to [syn: zest, spice, spice up]

The Free On-line Dictionary of Computing (19 January 2023):

ZEST

   An object-oriented extension of Z.

   ["Object Orientation in Z", S. Stepney et al eds, Springer
   1992].