To better understand the meaning of closed circuit, I made use of a circuit diagram below to illustrate the term. A closed circuit is the circuit that is complete. That is, current is flowing in the circuit and all the electronic components in the network are functioning. This happens when power supply or potential difference (Pd) is reaching and current is normally circulating across the circuit.
The below circuit diagram illustrates the term closed circuit’s meaning. The emphasis is the circle with an arrow pointing to show how a circuit or network becomes closed.
Read also the other terms (Short Circuit and Open Circuit) here: Short Circuit and its Meaning and The Meaning of Open Circuit respectively.