In digital logic, a don't-care term for a function is an input-sequence (a series of bits) that is known never to occur. The designer of a logic circuit to implement the function need not care about such inputs, but can chose the circuit's output arbitrarily, usually such that the easiest circuit results (minimization). Examples of don't-care terms are the binary values 1010 through 1111 (10 through 15 in decimal) for a function that takes a BCD value, because a BCD value never takes on such values; in the pictures, the circuit computing the lower left bar of a 7-segment display can be minimized to ab + ac + ad by an appropriate choice of circuit outputs for dcba=1010...1111.
"Don't care" may also refer to an unknown value in a multi-valued logic system, in which case it may also be called an X value. In the Veriloghardware description language such values are denoted by the letter "X". In the VHDL hardware description language such values are denoted (in the standard logic package) by the letter "X" (forced unknown) or the letter "W" (weak unknown).
An X value does not exist in hardware. In simulation, an X value can result from two or more sources driving a signal simultaneously, or the stable output of a flip-flop (electronics) 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.