Now that the Reagan administration has embarked on a global war against "terrorism," it becomes increasingly important to know what it is we are at war against. But instead of getting more precise, definitions are getting sloppier.

My own imprecision was brought forcefully to my attention by readers of a recent line in this space, in which I said that Shiite "terrorists" were killing Israeli occupiers of southern Lebanon. By phone and by letters, I was forcefully reminded that, unlike the PLO shelling of Israeli villages in the Galilee four years ago, the Shiites' attacks are directed against the soldiers of an illegal occupying force.

"The military activities among residents of south Lebanon against Israeli military forces correspond to classic tactics of guerrilla warfare against an occupation force in one's own country," said one reader.

Another reader asked: "If the Shiites were killing Soviets in Afghanistan, you would call them 'freedom fighters' -- so why the double standard where Israelis are involved?"

This was more than enough to encourage a search of definitions and case histories. Webster's starts out making it simple: "Terrorizing" means "to terrify": a highballing truck driver on Interstate 96 meets that test. But Webster went further: The "act of terrorizing" means "use of force or threats to demoralize, intimidate and subjugate . . . (especially) such use as a political weapon or policy." The battleship New Jersey standing off the Lebanese coast, even before it began shelling the Chuf mountains, meets that test. So does the CIA's mining of Nicaraguan ports or its blowing up of Nicaraguan refineries.

How about U.S. support of counterrevolutionary forces that are engaged, according to a flood of reliable reports, in atrocities of one sort or another involving civilians in the Nicaraguan countryside? The Reagan administration says the rebels are doing the same sort of things in El Salvador. Yet we call it "terrorism" in El Salvador while the Nicaraguan contras are called "freedom fighters." And the reader is right, up to a point, in his Lebanon/Afghanistan analogy. The techniques of resistance are the same: If "terrorism" fits one, it fits the other.

I am aware of the distinction between techniques and purposes and of the obvious differences in political objectives and/or ideological causes in all these cases -- as they are seen from the Reagan administration's upper reaches. From the standpoint of U.S. interests as defined by U.S. policy, there are good guys and bad, communists and anticommunists, noble and ignoble aims. Realpolitik, by these tests, requires a certain moral myopia.

But when a responsible Israeli official in Washington lumps PLO attacks aimed exclusively at innocents with the Arab oil embargo as "terrorism," and Jerusalem authorities talk about the recent reprisal raids against suspected Shiite hideouts in Lebanese villages as an "offensive defense" military operation, words begin to lose all meaning and myopia begins to get in the way.

Robert Kupperman, a Georgetown University authority on terrorism, concedes there is no all-purpose definition beyond "the use or threatened use of violence in the name of a political or ideological cause." That is to say, terrorism can be an extreme and repugnant expression of a legitimate grievance for which the aggrieved, rightly or wrongly, see no alternative recourse.

That is not an argument for respecting the Palestine Liberation Organization as a disciplined, responsible negotiating partner in Mideast peace talks. But not wishing to confront the underlying Palestinian grievance, the Israelis conveniently label the PLO a "terrorist organization."

Thus does this trigger word "terrorist" muddle clear-headed acceptance of the PLO as something to reckon with -- "an important actor with a degree of political reality," in Kupperman's words. Thus, as well, do imprecision and double standards confound diplomacy as the safe way out of the Arab-Israeli impasse.