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The Diac can be thought off as two thyristors connected in
antiparallel(A triac without a gate electrode):


         O        
  .......|.......
  :   .--*--.   :
  :   |     |   :
  :  _|_    | / :
  :  \|/   -+-  :
  :  -+-   /|\  :
  : / |    "|"  :
  :   |     |   :
  :   `--*--'   :
  :......|......:
         O

The diac only have two terminales. It will be triggered when the voltage over it rises high enough, and stay in the conducting state until the current flow through it falls bellow the holding current. The Diac is mostly used in triac triggering circuits. They may easily be mistaken for diodes, since the 'package' is similar, but a diac will have two rings, one in each end:
      ________
  ___| ||     |___ DIODE
     |_||_____|

      ________
  ___| ||  || |___ DIAC
     |_||__||_|

symbol: 

        |
      __|__
      \/ /\
      -----
        |
        |

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