In digital logic, a don't-care term[1][2] (abbreviated DC, historically also known as redundancies,[2] irrelevancies,[2] optional entries,[3][4] invalid combinations,[5][4][6] vacuous combinations,[7][4] forbidden combinations,[8][2] unused states or logical remainders[9]) for a function is an input-sequence (a series of bits) for which the function output does not matter.
Write-only registers, as frequently found in older hardware, are often a consequence of don't-care optimizations in the trade-off between functionality and the number of necessary logic gates.
In simulation, an X value can result from two or more sources driving a signal simultaneously, or the stable output of a flip-flop not having been reached.
In synthesized hardware, however, the actual value of such a signal will be either 0 or 1, but will not be determinable from the circuit's inputs.
It is sometimes possible that some states that are nominally can't-happen conditions can accidentally be generated during power-up of the circuit or else by random interference (like cosmic radiation, electrical noise or heat).