The Reagan administration is moving to put new restrictions on some of the costliest yet least debated federal benefit programs--those aiding the disabled.

It would do so mainly by redefining disability. The redefinition, if applied to both the welfare disability program, which the current administration proposal covers, and to the Social Security disability program, could eventually save the government--and cost disabled people--billions of dollars a year.

Disabilities are far more common in this country than is generally imagined. The government estimated four years ago that 21 million Americans--roughly one in 10--were disabled, half so severely that they said they could not work at all, the rest to the point that they could not do their former jobs.

In the late 1970s it was estimated that three out of 10 young males entering the work force would have at least one bout of disability before reaching retirement age. The Congressional Research Service recently estimated that programs to support and rehabilitate the disabled cost the government $44 billion in fiscal 1980, about $1 of every $12 the government spent that year.

The main disability program is part of Social Security. Social Security's disability trust fund now pays out about $17 billion a year, to about 4.5 million people.

There is also a large program for disabled veterans, and there is a welfare program--Supplemental Security Income or SSI--for the disabled who are not otherwise helped or who, even with other help, are still poor.

Reagan proposed sharp cuts in the Social Security disability program last year, as part of his effort to bobtail Social Security benefits generally. He withdrew the proposals after they stirred up a dust storm of protest.

This year, he is starting in on the smaller SSI program instead. The SSI disability program paid out about $4.3 billion in federal funds in 1981 to 2.2 million people.

Reagan would trim future benefits in two ways. First, he would require eligible applicants to have a disability that is expected to last 24 months instead of the current 12.

Second, he would redefine disability to make it much more a medical condition and less an occupational one than is now the case.

The administration says these proposals, which require congressional approval, would save $120 million in the next fiscal year.

And many observers believe that, if Congress adopts them for SSI, the administration will ultimately try to have them applied to the larger Social Security program as well.

These redefinitions come on top of a program, begun in the Carter administration, to examine disability applications more skeptically than before and reexamine every case every three years to make sure those receiving benefits are still entitled to them.

The Social Security Administration now reviews 500,000 disability cases a year, up from 150,000 a few years ago. Recipient groups complain that many beneficiaries are being wrongfully bumped off the rolls in the zeal to cut costs. There has been such a storm of protest, with reports that some people are being back-billed as much as $50,000 for overpayments, that several congressional committees are looking into the review process.

The trickiest part of the president's proposal has to do with the mix of medical and non-medical factors in defining disability. To qualify for benefits under the federal programs, an applicant now generally must be physically unable to perform, to any substantial degree, any kind of job.

But there is a kind of social overlay on this general rule. Workers can also qualify if, by virtue of age or lack of education or experience, it seems unlikely they can find work even though, physically, there is probably some job they could perform.

It is this bend in the general rule that the administration wants to curtail, in ways it has not yet spelled out. Older persons are the ones who benefit most from this exception. In 1979, according to the House Ways and Means Committee, 35 percent of all disability awards to persons 60 to 64 under Social Security were based in part on non-medical factors, but only 7 percent of awards to those under age 50.

What will happen to the president's proposals on the Hill is uncertain. The House Ways and Means subcommittee on Social Security has scheduled disability hearings later this month, but these will be looking partly at whether the stepped-up review process is too harsh.

The president may also have trouble because he has started with SSI. Critics point out that SSI is one of the safety net programs for the poor the administration has promised to preserve.