Definition
The reduction in electrical pressure (voltage) that occurs as current flows through a resistance in a circuit, such as a wire, connector, switch, or load. The amount of voltage drop across a component equals the current flowing through it multiplied by its resistance (Ohm's law: V = I × R). The sum of all voltage drops around a closed circuit equals the source voltage.
Plain English
When electricity flows through any part of a circuit that resists it, some of the voltage is used up at that point. That used-up amount is the voltage drop.
Context Anchor
Seen when troubleshooting aircraft wiring, batteries, starters, switches, lights, and other electrical equipment.
Derivation
Voltage' comes from the name of Italian physicist Alessandro Volta, who built the first electric battery. 'Drop' is used here in its everyday sense of a decrease — the voltage 'drops' from one side of a component to the other as electrical pressure is consumed pushing current through resistance.
Why Pilots Care
Excessive voltage drop can cause dim lights, slow engine cranking, or unreliable instrument and radio operation.
Analogy
It is like water pressure getting weaker as water moves through a narrow or partly blocked hose. The source may be strong, but less pressure reaches the end.
Grounding Statement
Picture water pressure falling as water squeezes through a partially clogged pipe — the pressure on the far side is lower than on the near side. Voltage drop is the electrical version of that pressure loss.
Intuition Check
Voltage drop does not mean all voltage is gone. It means the voltage is lower after electricity passes through part of a circuit.
Example Sentence 1
The technician measured a 2-volt drop across the battery contactor and replaced it, since a healthy contactor should drop only a fraction of a volt.
Example Sentence 2
High voltage drop in the starter circuit caused the engine to crank slowly on a cold morning.