voltage
Voltage is the electrical "push" that drives a current — the difference in electrical energy, per bit of charge, between two points, which makes electric charge want to flow from one to the other. Think of water in two tanks joined by a pipe: the bigger the difference in water level, the harder the water is pushed through. Voltage is that height difference for electricity. A 1.5 V battery gives a gentle nudge; a 230 V outlet shoves much harder.
Because voltage is a difference, it only exists between two points — there is no such thing as the voltage of a single, lonely point. That is why a bird on a high-voltage wire is unharmed: its two feet sit on nearby spots of the same wire, almost the same voltage, so there is barely any difference to push current through it. Connect a flashlight bulb across a battery's two terminals, though, and that 1.5 V difference drives just enough current to make it glow.
Voltage is always measured between two points; "the voltage here" really means "relative to a reference," usually the circuit's ground (taken as 0 V). It is named after Alessandro Volta, who built the first true battery around 1800. A common mix-up: high voltage is not the same as high danger — what harms you is the current that flows through your body, and voltage is only what pushes it there.