A combinational circuit is one where the output of that circuit at any time only depends on the present combination values of inputs. And the output is never depending on any past state value of input combination as well as the previous state output value.
Now if we take any example then the logic gate is the most basic building block of combinational logic. Now as we know logic gates is work according the specified logic for specified gate. And those combinational circuits which are made by logic gate maintain Boolean expression. In bellow see the block diagram of generalized combinational circuit.
In above block diagram we can see that combinational logic circuit has “n” inputs that mean it can take 2n combination of input values. The output “m” is depending on the Boolean expression of combinational logic circuit. Let take an example of a half adder circuit
We know in half adder circuit the number of input and number of outputs are two.
Those out puts are
And the logic relation maintain by both output are
SUM S = A.B^+A^.B
CARRY C = A.B
In bellow you will see the truth table, block diagram and logical circuit diagram of half adder.
From this above circuit and truth table we can easily understand that the output of this logic circuit is only depend on present combination value of inputs. So we can call it a combinational circuit.