For the time being, tri-state-logic may refer to logic circuits with at least one output adding a third, "high impedance" state (Z) to the binary "low" (0) and "high" (1) states as well as other contexts adding a third alternative.
In logic circuit implementation, tri-state refers to outputs/buffers that add a third, high impedance state, often denoted Z, to the binary low and high states.
In specification/design, a truth table may show don't care entries in addition to 0 and 1, conventionally denoted by a ×. As an example, an adder for two binary coded decimal digits does not need a specific output value for one input 1011 (11), as that is not a valid BCD code.
