An open circuit is a circuit path that is broken or incomplete. Because the path is not closed, current cannot flow through that part of the circuit.
What causes an open circuit?
Open circuits can be caused by a switch being off, a broken wire, a loose connector, a blown component, a cracked solder joint, or a missing connection on a breadboard or PCB.
Open circuit versus short circuit
A short circuit provides an unwanted easy path for current. An open circuit blocks current because the path is broken.
A multimeter continuity test is often used to find an open circuit.