The Collaborative International Dictionary of English v.0.48:

Emissary \Em"is*sa*ry\, a.
   1. Exploring; spying. --B. Jonson.
      [1913 Webster]

   2. (Anat.) Applied to the veins which pass out of the cranium
      through apertures in its walls.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Emissary \Em"is*sa*ry\, n.; pl. Emissaries. [L. emissarius,
   fr. emittere, emissum, to send out: cf. F. ['e]missaire. See
   Emit.]
   An agent employed to advance, in a covert manner, the
   interests of his employers; one sent out by any power that is
   at war with another, to create dissatisfaction among the
   people of the latter.
   [1913 Webster]

         Buzzing emissaries fill the ears
         Of listening crowds with jealousies and fears.
                                                  --Dryden.

   Syn: Emissary, Spy.

   Usage: A spy is one who enters an enemy's camp or territories
          to learn the condition of the enemy; an emissary may
          be a secret agent appointed not only to detect the
          schemes of an opposing party, but to influence their
          councils. A spy must be concealed, or he suffers
          death; an emissary may in some cases be known as the
          agent of an adversary without incurring similar
          hazard.
          [1913 Webster]

WordNet (r) 3.0 (2006):

emissary
    n 1: someone sent on a mission to represent the interests of
         someone else [syn: emissary, envoy]