President Reagan let the cat out of the bag last week when he accurately introduced Anthony M. Kennedy, his Supreme Court nominee of the month, as "a true conservative."

Among other things, the description reveals how broadly the president defines the word "conservative," which he also applied to two previous nominees who differ strikingly from Kennedy.

But Reagan is not alone in using the C-word loosely. That charge could be leveled with equal validity at conservative Republican senators delighted at the prospect of Robert H. Bork's judicial activism and resistant to Kennedy's record of judicial restraint. Most in the media, including me, are frequently guilty of loose political labeling. We have used the term "conservative" to describe everything from the Strategic Defense Initiative to a traditional life style.

Public opinion surveys by the Roper Organization over the last decade have found a fairly consistent pattern of political self-identification. Typically, the most recent poll finds that 43 percent of Americans consider themselves "conservative," 29 percent "moderate" and 23 percent "liberal." Reagan's political success has been based on an ability to gather the various conservatives under his tent, adding enough moderates to make a majority.

The skirmishing among Republicans over the Supreme Court nomination reveals the fragility of this coalition. A well-defined fault line already exists between older social conservatives with traditional values and younger economic conservatives who are largely libertarian in their social views. Withdrawn Supreme Court nominee Douglas H. Ginsburg, with or without marijuana, fits the values of the latter group.

Even supposedly like-minded social conservatives have many differences among themselves. The consensus is clearest on criminal justice, where Judge Kennedy's record is close to the American mainstream. Polls show that more than four of five Americans believe that courts treat criminals too leniently. A substantial majority favors capital punishment.

The agreement of Americans on these issues is why Reagan emphasized them in a speech written in anticipation of Kennedy's nomination. Instead, the president delivered it while nominating Ginsburg, who lacked criminal-justice experience. Now, with Kennedy, the argument at least fits the nominee.

Reagan's earlier attempt to put Bork on the high court revealed the downside of the conservative social agenda. A majority of Americans and of the Senate became convinced, unfairly or not, that Bork might revisit settled issues of civil rights and undo established abortion decisions. Most Americans are too conservative to want to take this risk.

In fact, the conservative social agenda is wildly controversial if criminal-justice issues are put aside. Abortion is emotionally divisive across the political spectrum. Most people oppose pornography but are uneasy with the implications of rigid censorship. On these difficult issues and many others, there is no conservative consensus and no national one.

Reagan has often assuaged social conservatives while keeping their more controversial advocacies on the back burner. He endorses the antiabortion cause while avoiding antiabortion rallies. He gives lip service to prayer in schools and lets it go at that. But he has appointed many judges who recognize that the policeman's lot is not an easy one and who hold as strict a view as his on dealing harshly with criminals.

Last week, when a chastened Reagan confessed that he had become "a little bit wiser" and finally nominated Kennedy, the extra-chromosome conservatives accused him of betraying his principles. Richard Viguerie, for one, accused the president of "a total surrender to the left."

What Reagan really did was retreat, not surrender. He retreated to the social issue that he cares most about, criminal justice, and on which he has a fairly reasonable record. Reagan, no lawyer, is concerned about crime victims. He believes that judges should "interpret the law, not make it," as if that distinction were always clear. It is an uncomplicated view both widely shared and profoundly conservative.

Reaganism of the Week: Asked last Tuesday if Attorney General Edwin Meese III were an "embarrassment," the president said: "He's no embarrassment to me. I've known him for 20 years, and I've found him of sound mind and great loyalty and capability in all that time."