IN the cruel calculus of deaths and injuries, the explosion that rocked New York's financial district last week was not nearly the catastrophe it might have been. More people were killed in a single, execution-style slaying in a Bronx apartment a week earlier than died in the blast. But the economic and psychological impact of the World Trade Centre bombing, like shock waves, has continued to spread, sending a long cold chill down the American spine. Official estimates as to when hundreds of businesses and their 55,000 employees can return to the empty twin towers grow more pessimistic by the day. At first it was a matter of days, then a week. But the damage is profound. Whether by design or by accident, the ton of dynamite - if that was the material used - that ripped through seven subterranean floors was perfectly placed to knock out virtually all the complex's communications, security and electrical networks. Now some experts are worried about structural integrity, and project a wait of several weeks before the buildings reopen. In the meantime, dozens of big banking, accounting, brokerage and legal firms exiled from this financial and commercial nerve centre are scrambling to recover vital data, find temporary office space and serve their customers. One Japanese bank figures tolose US$20 million for every day they are unable to conduct business. On Monday, the Governor of New York declared the site an economic disaster area, as if it had been hit by a hurricane. This disaster, however, was not an act of god but an act of man, which prompts two burning questions: Who? Why? The FBI, which is helping the New York police to investigate the bombing, has been stingy with information, but all the little bits and pieces collected so far seem to point in the same direction. Terrorism. Terrorism is a familiar spectre to Americans. Many remember, as if it happened yesterday, the 1983 bombing of a US Marine barracks in Beirut that left 241 soldiers dead, or the 259 passengers and twisted wreckage of Pan Am flight 103 strewn about the Scottish countryside near Lockerbie. But somehow - and no one knows quite why - terrorism has rarely struck at home. The last major terrorist bombing on US soil was at La Guardia Airport in New York nearly 20 years ago, for which a Puerto Rican independence group remains the prime suspect. Over the past five years, terrorist incidents of any size have dropped from ninein 1988 to only two last year, the FBI says. America has been relatively immune to the scourge that has plagued every major Western European capital. The prospect of terrorism in the US seems especially shocking now because the reality of a new world disorder has not yet dispelled the mirage of a post-Cold War peace. But, even more than the chaos in Somalia or the strife in what was once Yugoslavia, this single explosion in the heart of Manhattan - should if prove to be the work of international terrorists - will startle Americans into understanding that no place is safe. President Clinton, sensing the impact on the national psyche, offered reassurance the day after. ''Americans should know we'll do everything in our power to keep them safe in their streets, their offices and their homes,'' he said. If others ask whether terrorism is poised to visit American shores, however, I find myself asking another question. Why don't we recognise the terrorism that is already here? If the death of five people in an office building in New York sets the nation on edge, why do we sigh with resignation when an entire family is gunned down in their home? Why do we call the former an act of ''terrorism'' that threatens the American way of life, and the latter an act of ''urban violence'' that is the American way of life? There are two components to terrorism: the crime and the reaction. A ''successful'' terrorist action depends on creating an atmosphere of anxiety, the fear that random violence can strike anywhere, any time. Without that, it does not work. But if terrorism terrifies because the victims could just as well be picked from a telephone directory, then America is already beset by it. The drive-by shootings of inner-city gangs, the stray-bullets of urban gun battles or the lethal outbursts of well-armed sociopaths - none of these quintessentially American phenomena, it seems to me, is any less random than blowing up a world-famous office building. Nor are they any less terrifying, at least for the lawful people who live in their midst. and yet, wedo not call it by its proper name. Random violence without a ''cause'', some will argue, is not really terrorism, even if it produces the same effect. The fear of earthquakes, after all, is terrifying too. True, but does the kind of home-grown violence reported daily in the local news lack a ''cause''? Does it not sometimes tap into a deep reservoir of unarticulated rage, a feeling of powerlessness, a sense of oppression? These are not, of course, justifications for violence. But they are a caution. Perhaps we should be as alarmed about what we don't call terrorism and as by what we do.