a voltage source

A voltage source is anything that tries to hold a fixed voltage across its two terminals, no matter what you connect, the pump that keeps a constant pressure. A battery, a bench power supply, a USB port and a wall adapter are all voltage sources. A 5 V source promises 5 V between its plus and minus, and it will supply whatever current the circuit asks for to keep that promise, within its limits. It sets the pressure; the circuit decides the flow.

An ideal voltage source holds its voltage perfectly at any current, but real ones cannot. Every real source has a small internal resistance, so its voltage sags a little as you draw more current: a fresh AA battery reads 1.5 V doing nothing but may droop to 1.3 V under a heavy load. This internal resistance is why a nearly dead battery still shows full voltage on a meter (no current) yet collapses the moment it has to deliver. Pull too much and the voltage falls, the source heats, or a protection trips.

Most circuits run from a voltage source, and most of the foundations assume one: you set up a known voltage, and Ohm's law tells you the current each part will draw. When you see a battery symbol or a Vcc rail on a schematic, that is a voltage source. Its opposite, a current source, holds the current instead, less common as a power outlet but essential inside many circuits.