Source : Webster's Revised Unabridged Dictionary (1913)

Nod \Nod\, v. i. [OE. nodden; cf. OHG. kn?t?n, genuot?n, to
   shake, and E. nudge.]
   1. To bend or incline the upper part, with a quick motion;
      as, nodding plumes.

   2. To incline the head with a quick motion; to make a slight
      bow; to make a motion of assent, of salutation, or of
      drowsiness, with the head; as, to nod at one.

   3. To be drowsy or dull; to be careless.

            Nor is it Homer nods, but we that dream. --Pope.

Nod \Nod\, v. t. [imp. & p. p. {Nodded}; p. pr. & vb. n.
   {Nodding}.]
   1. To incline or bend, as the head or top; to make a motion
      of assent, of salutation, or of drowsiness with; as, to
      nod the head.

   2. To signify by a nod; as, to nod approbation.

   3. To cause to bend. [Poetic]

            By every wind that nods the mountain pine. --Keats.

Nod \Nod\, n.
   1. A dropping or bending forward of the upper oart or top of
      anything.

            Like a drunken sailor on a mast, Ready with every
            nod to tumble down.                   --Shak.

   2. A quick or slight downward or forward motion of the head,
      in assent, in familiar salutation, in drowsiness, or in
      giving a signal, or a command.

            A look or a nod only ought to correct them [the
            children] when they do amiss.         --Locke.

            Nations obey my word and wait my nod. --Prior.

   {The land of Nod}, sleep.

Source : WordNet®

nod
     n 1: a sign of assent or salutation or command
     2: the act of nodding the head
     [also: {nodding}, {nodded}]

nod
     v 1: express or signify by nodding; "He nodded his approval"
     2: lower and raise the head, as to indicate assent or agreement
        or confirmation; "The teacher nodded when the student gave
        the right answer"
     3: let the head fall forward through drowsiness; "The old man
        was nodding in his chair"
     4: sway gently back and forth, as is in a nodding motion; "the
        flowers were nodding in the breeze"
     5: be almost asleep; "The old man sat nodding by the fireplace"
     [also: {nodding}, {nodded}]