An explosion is a violent expansion of some sort, a bursting out of gas, air, fuel, or other substances. An implosion is a similar bursting but one that is directed inward. When a gas tank explodes, debris is hurled outward; when it implodes, walls collapse and fall inward. A similar distinction is found in the words eruption and irruption (See eruption).
Dictionary of problem words and expressions. Harry Shaw. 1975.