Source : Webster's Revised Unabridged Dictionary (1913)

Tick \Tick\, n.
   1. A quick, audible beat, as of a clock.

   2. Any small mark intended to direct attention to something,
      or to serve as a check. --Dickens.

   3. (Zo["o]l.) The whinchat; -- so called from its note.
      [Prov. Eng.]

   {Death tick}. (Zo["o]l.) See {Deathwatch}.

Tick \Tick\, v. t.
   To check off by means of a tick or any small mark; to score.

         When I had got all my responsibilities down upon my
         list, I compared each with the bill and ticked it off.
                                                  --Dickens.

Tick \Tick\, n. [Abbrev. from ticket.]
   Credit; trust; as, to buy on, or upon, tick.

Tick \Tick\, v. i.
   1. To go on trust, or credit.

   2. To give tick; to trust.

Tick \Tick\, n. [OE. tike, teke; akin to D. teek, G. zecke. Cf.
   {Tike} a tick.] (Zo["o]l.)
      (a) Any one of numerous species of large parasitic mites
          which attach themselves to, and suck the blood of,
          cattle, dogs, and many other animals. When filled with
          blood they become ovate, much swollen, and usually
          livid red in color. Some of the species often attach
          themselves to the human body. The young are active and
          have at first but six legs.
      (b) Any one of several species of dipterous insects having
          a flattened and usually wingless body, as the bird
          ticks (see under {Bird}) and sheep tick (see under
          {Sheep}).

   {Tick bean}, a small bean used for feeding horses and other
      animals.

   {Tick trefoil} (Bot.), a name given to many plants of the
      leguminous genus {Desmodium}, which have trifoliate
      leaves, and joined pods roughened with minute hooked hairs
      by which the joints adhere to clothing and to the fleece
      of sheep.

Tick \Tick\, n. [LL. techa, teca, L. theca case, Gr. ?, fr. ? to
   put. See {Thesis}.]
   1. The cover, or case, of a bed, mattress, etc., which
      contains the straw, feathers, hair, or other filling.

   2. Ticking. See {Ticking}, n.

Tick \Tick\, v. i. [imp. & p. p. {Ticked}; p. pr. & vb. n.
   {Ticking}.] [Probably of imitative origin; cf. D. tikken, LG.
   ticken.]
   1. To make a small or repeating noise by beating or
      otherwise, as a watch does; to beat.

   2. To strike gently; to pat.

            Stand not ticking and toying at the branches.
                                                  --Latimer.

Source : WordNet®

tick
     n 1: a metallic tapping sound; "he counted the ticks of the
          clock" [syn: {ticking}]
     2: any of two families of small parasitic arachnids with barbed
        proboscis; feed on blood of warm-blooded animals
     3: a mark indicating that something has been noted or completed
        etc.; "as he called the role he put a check mark by each
        student's name" [syn: {check mark}, {check}]
     4: a light mattress

tick
     v 1: make a clicking or ticking sound; "The clock ticked away"
          [syn: {click}]
     2: make a sound like a clock or a timer; "the clocks were
        ticking"; "the grandfather clock beat midnight" [syn: {ticktock},
         {ticktack}, {beat}]
     3: sew; "tick a mattress" [syn: {retick}]
     4: put a check mark on or next to; "Please check each name on
        the list"; "tick off the items" [syn: {check}, {check off},
         {mark}, {mark off}, {tick off}]

Source : Free On-Line Dictionary of Computing

tick
     
        1. A {jiffy} (sense 1).  2. In simulations, the discrete unit
        of time that passes between iterations of the simulation
        mechanism.  In AI applications, this amount of time is often
        left unspecified, since the only constraint of interest is the
        ordering of events.  This sort of AI simulation is often
        pejoratively referred to as "tick-tick-tick" simulation,
        especially when the issue of simultaneity of events with long,
        independent chains of causes is {handwave}d.  3. In the FORTH
        language, a single quote character.
     
        [{Jargon File}]