A transient is a "momentary change in the voltage or current over a very short time. This short time interval is less than 1 cycle, or 16 milliseconds. More often, transients are measured in microseconds rather than milliseconds. Frequently this transient is called a voltage "Spike."
The transient has a separate beginning and end. It may occur often or even at regular intervals, but it will have a beginning and end.
Another aspect of transients is whether they're "impulsive" or "oscillatory" which refers to the shape of the transient. If the transient occurs, and the voltage returns to its full value, it's impulsive. If, however, the transient bounces around, getting smaller at each bounce, we call it an oscillatory transient because it "rings," or oscillates.
According to the CBEMA curve, transients usually have to be greater than 100% of the voltage to be a problem. That is why most experts feel transients that peak at 50 volts or so are no big deal.
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