This appears to be a term that gets used and abused so I thought I'd start off with a few thoughts in no particular order.
Surge is a highly ambiguous term that describes a high voltage event or transient voltage that in turn can cause damage to electrical items and appliances etc. It's a term that seems to get applied all too often as blanket excuse that's used in order to avoid having to define the actual event or cause of the event.
Surge is a term that's often incorrectly used to describe an overvoltage event that's caused during cable theft. When a neutral wire is cut in a three phase supply before the phase wires are cut, the voltage on the neutral is no longer held at zero volts. The neutral then 'floats' at something approaching the voltage of the phase with the highest load at the time. Most domestic properties have a single phase supply which originates from one phase of a 3-phase supply plus a neutral. It's pot luck amongst the properties in the area of the theft which ones will recieve a high voltage and which ones will receive close to zero volts but there's a good chance that two thirds of the properties will suffer from electrical items being damaged by over-voltage.
This type of overvoltage fault would not be considered electrically as a 'surge' although insurance companies might beg to differ if there's a surge clause in their policy.
For me, classing every instance of overvoltage on an electrical network as a surge is like a coroner classing every cause of death he investigates as being due to cardiac and respiratory failure. Yes, the guy that slams his motorcycle at 240Kmh into a concrete piling suffered heart and breathing failure but that's not what will be listed as actual cause of death. Similarly I'm not keen on the blanket term 'surge' being applied as the cause of every electrical failure by high voltage.