In a closed circuit the impedence become zero at a given point.

I would add some more ideas to Surge's definition.

Most of the time, short circuit happen when the signal line or the power line (in an electrical/ electronics ckt) come into contact with the ground or return line. This, as indicated by Surge, give rise to a zero impedence resulting in heavy current demand.

Also, there are possibilities that two signal lines come into contact. This also is termed as a short circuit. (Suppose there is an input trigger signal coming into a delay circuit. After a fixed delay, the signal is transferred to a signal processor. If the input to the delay ckt comes in contact with its output, the delay module has no effect. We say, the delay module is short-circuited.)

It's not that a short circuit is always undesirable. Circuit trouble shooters, at times, find the cause of malfunctioning by bypassing(short-circuiting) intermediate stages; power supply units trigger forced short circuit of supply lines (and thereby causing the fuse to blow off) when abnormalities are detected in the circuit while functioning.

Narayanan UM
umn@ieee.org

Reference: IEEE Standard 100 "IEEE Dictionary of Electrical and Electronics Terms"
The above reference lists several definitions depending on a context, which you might look into.
One of them reads: An abnormal connection of relatively low resistance, whether made accidentally or intentionally, between two points of different potential in a circuit.

Another aspect of short circuits is that they are not really zero ohms because the involved metals are not superconductors. That can be useful when troubleshooting circuit boards where copper foil runs can be very long, even running to the full physical span of the board.

Once you've discovered that two traces are shorted, you may be able track that short to its location using a digital ohmmeter which can resolve milliohms. You measure the milliohms impedance between the two foils and gradually change the meter probes' points of contact on those foils until you find the minimum milliohms value. When you have done that, your probes will be physically as close to the actual point of the short circuit as they can get and you can do a visual inspection in that immediate area to finally find the fault itself.

It may take some patience to do this, but I've seen many circuit boards rescued from having to be scrapped.

John Dunn
Ambertec, Inc.

When two conductive parts at different potentials touch, then you have a short circuit.
Once they touch, they are forced to the same potential (at that point), and to compensate for this new forced potential, a new current (aka short circuit current) will develop.