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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.