Circuit
A circuit is a closed loop of conductors through which electric current can flow, usually containing a source, a load, and connecting wires.
Definition
A circuit is a complete loop that electricity can travel around, needing a power source such as a battery, wires, and something to run like a bulb or motor, and a switch is just a gap that can be opened and closed: on closes the loop so current flows, off opens it so current stops. An electric circuit is a closed path through which charge flows; every working circuit has a voltage source, conductors, and one or more loads that convert electrical energy into other forms, and loads can be arranged in series along one path or in parallel across multiple paths, with an open circuit having a break that carries no current. Circuit analysis rests on Kirchhoff's laws, the junction rule ($\sum I_{\text{in}} = \sum I_{\text{out}}$, charge conservation) and the loop rule ($\sum \Delta V = 0$ around any closed loop, energy conservation), combined with element laws such as $V = IR$ for resistors, $Q = CV$ for capacitors, and $V = L\,dI/dt$ for inductors; real sources have internal resistance, which limits maximum power transfer to the case where load resistance equals source resistance, and time-dependent circuits obey differential equations with characteristic time constants $\tau = RC$ or $L/R$.
Example
Connecting a battery, two wires, and a bulb into a loop lights the bulb, and disconnecting one wire breaks the loop and darkens it; a $6\ \text{V}$ battery connected to a $2\ \Omega$ bulb drives $I = 6/2 = 3\ \text{A}$, converting $P = IV = 18\ \text{W}$ into light and heat, and adding a second identical bulb in series drops the current to $1.5\ \text{A}$ while adding it in parallel keeps each bulb at $3\ \text{A}$. A $12\ \text{V}$ source with $1\ \Omega$ internal resistance feeding a $5\ \Omega$ resistor gives $I = 2\ \text{A}$ from $12 - I(1) - I(5) = 0$, delivering $20\ \text{W}$ to the load while wasting $4\ \text{W}$ internally, and swapping in a $1\ \Omega$ load matched to the source resistance gives $I = 6\ \text{A}$ and the maximum possible $36\ \text{W}$ delivered.
Key Insight
A short circuit is a loop with almost no resistance, so current becomes enormous and wires overheat, which is exactly what fuses and circuit breakers are built to interrupt. A circuit is really an energy-transport system with two conservation laws baked in: the battery is the only place energy enters, every other element stores it briefly or turns it into heat, light, or motion, and the loop rule is just the ledger balancing.