Lightning is an electrical discharge in the atmosphere, very similar to a spark.
It is the electrical breakdown of insulating air to provide a transient, conductive path along which a current can run to neutralize the charge. Lightning mostly occurs in thunderstorms. Other situations in which lightning occurs are dust storms, blizzards, volcanic eruptions and nuclear explosions. It seems to be a phenomenon occuring in large volumes of particle clouds when they are being transported in the atmosphere. In thunderstorms, charge is being separated by what is thought of being several processes.