WHEN the outrage over Love Canal was at its height, more than 20 years ago, hundreds of families had to be evacuated from their homes after 21,000 tons of chemicals buried beneath them started oozing into their basements and contaminating their groundwater.

Today, families are once again settled in the same neighborhood in upstate New York, now rechristened Black Creek Village. They live in neat, new ranch houses and federal officials recently announced that they now consider this notorious symbol of industrial pollution clean.

But what does clean mean when the pollutants that rendered Love Canal dangerous to humans remain exactly where they were? In fact, there is no accepted standard, and clean, in practical terms, often means still polluted -- but in a different and less dangerous way.

The Environmental Protection Agency, for example, has deleted 278 sites from the 1,200 on the Superfund national priorities list (the fund itself was created partly in reaction to Love Canal). Each has been defined as clean in a different way, and with few exceptions the offending pollutants were never removed.

What makes the notion of clean so slippery is the relative newness of the idea of decontaminating industrial sites and the unpalatable truth that treating pollution, even rendering it harmless, almost never means getting rid of it. The sin, once committed, cannot be entirely undone, and this is something no one really wants to hear.

Politicians ''can't politically make the jump to telling the public that they have to accept a certain amount of contamination,'' said Michael B. Moore, an environmental consultant from Vermont who is chairman of the Superfund task force of the National Ground Water Association, a professional group with a special interest in cleaning up contaminated sites.

When federal officials put Love Canal on the Superfund list, some residents thought they knew what clean meant.

''We were led to believe that they were going to go in with bulldozers, take 20,000 tons of waste out of Love Canal and clean up the neighborhood so we could live there,'' said Lois Marie Gibbs, whose home had to be demolished because the ground beneath it dripped with chemicals.

Never having undertaken a project like Love Canal, federal officials had no idea how much contamination was buried there until they started testing the soil. They quickly realized that the volume of hazardous waste was enormous, and that removing the rusting and dented chemical drums was riskier than leaving them there.

Then, in a pattern followed at many other sites, the government and the Hooker Chemical Company (now the Occidental Chemical Corporation), capped the chemical swamp with a thick layer of clay, installed pumps and drains to control runoff and ripped up miles of contaminated sewer pipe. The chemicals themselves were left in the ground, surrounded by a cyclone fence.

Jane M. Kenny, the E.P.A. regional administrator, insisted that no standards were lowered in removing Love Canal from the Superfund list. Even though the chemicals haven't been removed, she said, the $400 million cleanup has contained the pollution and reduced the health risks, which is the cleanup standard the agency aims for.

''I know that saying clean makes people crazy,'' she said, ''but in terms of Love Canal, the area is now protective of the environment, the site is contained and we believe that we have eliminated the exposure.''

The chasm between the government's definition of clean and community expectations hasn't narrowed in 20 years.

''If she says Love Canal's cleaned up, that's just a blatant lie,'' said Ms. Gibbs, now executive director of the Center for Health, Environment and Justice, which works with communities facing environmental problems.''

When Congress established the Superfund in 1980, it deliberately refrained from setting a single standard, insisting only that the E.P.A. protect health and the environment in a cost-effective way.

The gold standard was a level of cleanliness where there was only a one in a million chance that there would be more cancer in the area than normal. But that was not a practical goal at many cleanup sites, including Love Canal, where the level of risk of additional cancers is now reckoned at one in 10,000.

The E.P.A. will not certify a site as clean in which the risk of additional cancers exceeds one in 10,000.

The term brownfields is another way of defining clean and saying the politically unspeakable -- that a certain amount of contamination will always be with us. Brownfields are former industrial sites that are cleaned just enough so the remaining risk is compatible with the way the land will be reused. Owners get to define clean one way if they intend to build, say, a parking lot, and another if they plan to build homes.

It is yet another way of saying clean, and still polluted.