A jump start is a colloquial term for a method of starting an automobile or other internal combustion engine-powered vehicle having a discharged battery.
Most U.S. passenger vehicles use a standard 12-volt direct current (DC) Lead-acid battery which, when the driver turns the ignition key, an electric motor engages the teeth on the flywheel, briefly turning the flywheel which is connected to the crankshaft and powers the spark plugs until ignition is achieved and the engine can produce its own electrical power from its generator or alternator.
When a battery fails or is discharged, such as by inadvertently leaving one's headlights switched on while parked, the car's engine will not "turn over" when the ignition key is turned. Many motorists carry "jumper cables" which consist of a pair of heavy gauge wires with large crocodile clips at each end.
An automobile with a good battery is parked near the car needing the jump start and the cables are attached in this order:
A "safety" jumper cable, in two pieces. The red and black leads of each piece are connected to a car's battery, and then the yellow connectors are plugged together.
With a safety cable, such as the one shown in the picture, the two pieces are connected together. The safety cable has color coded crocodile clips and the connectors are polarized so that they can only be inserted in the correct orientation.
Since the entire engine block is grounded to the negative terminal, the cable need not be connected directly to the dead battery's negative terminal, and in fact it would be unwise to do so as sparks from the connection (when the circuit is completed) could ignite the battery. (Highly explosive hydrogen gas can be given off by a battery, though this is less of a risk with the completely sealed "maintenance-free" batteries unless their case is compromised). A good connection point would be a piece of unpainted metal at least eighteen inches away from the dead battery. Some engines have eyelets which are used to attach chains when the engine needs to be lifted out of the car; these make good connection points for the jumper cable.
(Many auto manufacturers specify methods of jump-starting their cars, such as a different sequence of attaching or detaching jumper cables. For your safety, please read your owner's manual!)
The "good" car is then started, and the dead battery is allowed to charge for a few minutes. Then the car with the dead battery can be started, the cables carefully detached, and the formerly-stranded motorist goes on his or her way. If the original cause of the dead battery was simply a drain such as the headlights being left on overnight, then the car's generator should take care of finishing the recharge and keeping the battery charged; but if the battery is damaged or old or there is some problem with the car's electrical system, then the motorist should keep his engine running until he can buy a new battery or reach a service station.
Note that unlike a battery charger, a car's alternator lacks the current-limiting circuit needed for the proper trickle-charging of deeply discharged batteries, and should not be used for that purpose. Charging a battery at too high a current is not good for either the alternator, or the battery.
In localities or situations lacking in Good Samaritans, there is always the auto club, for its members. Roadside assistance vehicles, dispatched by them or not, can be expected to carry equipment at least as handy as a self-contained and portable jump-start unit consisting of a battery and two leads to connect it to the stopped car's starter circuit, substituting for the dead battery.