In the electrical trades, the terms "line" and "load" are shorthand words that refer to the wires that deliver power from the source to a device (line), vs. those that carry power onward to other devices further along the circuit (load). A number of other more conversational terms are also used to describe the same thing, such as incoming vs. outgoing wires or upstream vs. downstream.
The terms are used in the context of a single device and electrical box, so that the wires that deliver power into the box are described as the line wires, the upstream wires, or the incoming wires, while the wires passing onward to other devices are described as load, downstream, or outgoing wires. And these terms are relative to the location of the device in the circuit, since the load wire for one outlet becomes the line wire for the next receptacle downstream in the circuit.
The terms line and load have a number of applications at different locations in an electrical system.
Service Entry and Main Panel
The incoming feed from the utility company comes into the line side of an electric meter. It leaves the meter from the load side and then feeds the line side of a disconnect or electrical service panel. The service panel also has line and load connections—the line feeds the main breaker in the panel, while the individual branch circuit breakers can be considered the load, with respect to the main breaker.
Outlets (receptacles), switches, light fixtures, and other electrical devices typically are wired in multiples on a single circuit. With the first device, the line is the wire running from the service panel to the device, and the load is the wire running from the first device to the second device downstream on the circuit. At the second device, the line is the power source coming in from the first device; the load is the wire going out to the third device on the circuit, and so on.
The same meaning can apply to the device itself. The line side of an outlet is where you connect the incoming source power. The load side is where the power leaves the device (or electrical box) and travels down the circuit.
Line and load have special meaning when wiring ground-fault circuit-interrupter (GFCI) outlets. GFCIs have two pairs of screw terminals for connecting wires: One pair is marked LINE, and one is marked LOAD. Connecting to the line terminals only results in the outlet providing GFCI protection only for that outlet. Connecting for the line and the load terminals (using two electrical cables or two sets of pigtail wires) provides GFCI protection for that outlet as well as for other standard outlets located downstream on the same circuit.
Other Meanings of Line and Load
When wiring low-voltage circuits, such as those supplying doorbells or landscape lights, "load" refers to the parts of the circuit that are at full household voltage (usually 120 volts), to distinguish them from the low-voltage wiring and devices that are used after the voltage is stepped down at a transformer.
Load is also a general term to describe the electrical demand, or power draw, that a device or appliance places on a circuit. For example, on a lighting circuit, you can add up the maximum wattage of all of the light fixtures on the circuit to calculate the "total load," or maximum potential power demand of all the lights.