Source : Free On-Line Dictionary of Computing

tri state
     
         A feature of some {digital} electronic devices
        which allows a connector (pin) to either act as a normal
        output, driving a signal onto a line, or to be "tri stated" -
        set to a high-impedance ("high Z") condition.  This allows
        other outputs to drive signals onto the line.  Often the same
        connector also functions as an input when its output circuitry
        is tri stated.
     
        Tri-state outputs are typically used for the connection of
        several digital circuits to a shared {bus} onto which any one
        of them may output data for the others to input.
     
        (1996-07-26)