A hot line (also known as a phase line) is a wire in the latter stages of the distribution grid (like inside your house) that has a non-zero average voltage relative to the Earth (also called ground), as opposed to neutral lines, which are ideally at ground potential. Since hot lines carry electricity that has a high potential energy, they are shock hazards. Many electrical devices minimize this risk by taking advantage of polarized electrical outlets (see figure 1) to ensure that the on/off switch is on the hot line, effectively limiting the length of the hot line, which minimizes the risk of shock as only a relatively small segment of the wiring (the wire before the switch, as opposed to the whole device) is considered "hot" when the circuit is open.
For a more in depth discussion, including circuit diagrams please see All about circuits.
For further information please see the related pages below: