Define combinational logic circuit

A combinational logic circuit is a digital circuit whose outputs at any instant depend only on the present combination of its inputs. It has no memory and no feedback, so past inputs do not affect the output.

           +--------------------+
 x1 ──────>|                    |──────> z1
 x2 ──────>|   Combinational    |──────> z2
  :        |   logic (gates)    |   :
 xn ──────>|                    |──────> zm
           +--------------------+
     n inputs                 m outputs

Key features

• Built only from logic gates (AND, OR, NOT, NAND, NOR, XOR).
• Output = f(present inputs); it can be fully described by a truth table or Boolean expressions, one per output.
• No clock and no storage elements (no flip-flops).
• An n-input circuit has $2^n$ input combinations.

Examples: half adder, full adder, subtractor, multiplexer, demultiplexer, encoder, decoder, magnitude comparator, code converters.

By contrast, a sequential circuit contains memory (flip-flops), so its output depends on both present inputs and past history (the present state).