At its most basic, a “circuit” is a signal path through which an electrical current can flow. A circuit must be a complete and “closed” path, meaning that the current circulates around the circuit and can return to the beginning.
A circuit is composed of multiple electronic components, such as resistors, transistors, capacitors, inductors, and diodes that have been interconnected. Circuits can be constructed of discrete components connected by individual pieces of wire, but today it is much more common to create interconnections using photolithographic techniques to create a printed circuit board and soldering the electronic components to the resulting interconnections to create a finished circuit. In an integrated circuit, the components and interconnections are formed on the same substrate, typically a semiconductor such as silicon.