New Year, new you. January often starts with resolutions about self-improvement of mind and body. For many, that can mean an embrace of clean beauty, in its myriad forms.

Swiss glacier water to remove your makeup? La Prairie can give you that for $120. Intrigued by the possible regenerative powers offered by the microbes in Finnish forest mulch? Snap up a cleansing cake from Luonkos for 33 euros (about $35). Curious about algae and sea kale sunscreens, vegan lipsticks or gritty exfoliating soaps made from spent coffee grounds? Look around and it seems as if more and more consumers are jumping on a beauty bandwagon that promises clean skin — and an even cleaner conscience.

The research consultancy Brandessence estimates that nearly one-third of the United States market is now labeled clean, with an increase of 12 percent expected from 2020 to 2027. Currently, clean beauty has 5.6 million hashtag views on Instagram and 1.2 billion on TikTok.

And many brands are jostling for a place in the market, among them indie start-ups like Merit and Saie Beauty and major luxury names like Dior, which released its first alcohol-free, water-based perfume, and Stella McCartney, fashion’s eco-queen, who introduced a natural origin skin-care line.

But what does clean beauty actually mean?

“If you ask 10 different people what clean beauty means, you’ll get 10 different answers,” said Caroline Hirons, a prominent British skin-care influencer. When you scrape away at it, she said, it “doesn’t really mean anything.”

Much like the murky term “sustainability” in fashion, there is no clear definition of clean beauty — and no consensus on the specific substances and chemicals that should be avoided or embraced. As awareness of the lack of regulation in the beauty industry has risen in recent years, so too has skepticism about the “clean” movement.

But growing faster still is the global popularity of clean consumerism, as shoppers gravitate toward marketing terms like “naturally derived,” “cruelty free” and “nontoxic” when it comes to what they put in — and on — their bodies (with the notable, and curious, exception of injectables like Botox).

Where did the term ‘clean beauty’ come from?

Although skin-care brands like Origins and Aveda, early adopters of a “natural” vocabulary, appeared in the late 1980s, the consensus is that clean beauty emerged from Southern California in the 1990s, alongside the “clean eating” trend.

As many consumers became preoccupied with notions of wellness, some beauty companies began to promote products as nontoxic, safe and natural. Yet there has never been a set of legal guidelines governing the use of such terms.

Currently, the European Union bans more than 1,300 ingredients from use in cosmetics (though many would rarely be found in personal care items). In the United States, the Food and Drug Administration bans 11 cosmetic ingredients. Last fall, Congress introduced the Safer Beauty Bill Package, which, if passed, will codify legal definitions for terms like “natural” and “naturally derived” and ban ingredients like parabens and formaldehyde. Japan, another major beauty market, has different regulatory standards,.

This means that “many brands are taking it upon themselves to define clean beauty according to their ideals and agendas,” said Akshay Talati, the vice president for product development in the Goop beauty and wellness division.

Then again, there are brands that don’t want to be tarnished by the “clean” association.

“I think ‘clean’ skin care is all a load of bollocks,” Ms. McCartney told Elle UK magazine last year when she introduced Stella, her skin-care line. She said she understood why people use the word, “because it conjures up wonderful images of purity, but I would never use it.”

So how is it defined?

Tata Harper is widely considered a godmother of the clean beauty movement, with a cult brand of the same name. She grew up in Colombia, where she watched her grandmother make body scrubs and hair masks from ingredients sourced at her local market, and later trained as an industrial engineer.

Ms. Harper started her brand in 2007, and her products use ingredients like antioxidant-rich witch hazel, hydrating jasmine and plumping alfalfa extract. A 30 milliliter bottle of her elixir vitae serum, with barley juice, borage leaf and sea buckthorn, costs about $490.

“At the time, natural skin care was not really made for a serious skin-care client like myself,” Ms. Harper said. “That’s when I realized I had to create my own line because there were no options.”

Goop, the lifestyle empire founded by Gwyneth Paltrow, is one of the movement’s most vocal advocates. It defines clean beauty as “products made without ingredients shown or suspected to harm human health or that of the planet.” On its website, Goop sells products under its own name and from others, tested, it says, by its own scientists, toxicologists and regulatory experts for ingredients that are carcinogenic, or are irritants or hormone disrupters.

“Goop prioritizes ingredients that are ethically sourced, of nonanimal origin and cruelty free, and we use sustainable or renewable bio-based sources wherever possible,” Mr. Talati said.

In an email, Ms. McCartney said that her new skin-care line had been developed over three years and ruled out more than 2,000 ingredients, with the guidance of Quantis, a company specializing in environmental sustainability.

The products are made from ingredients that are at least 99 percent natural, like lingonberry extract (to support elasticity and firmness) and wild harvested fulse algae (to reduce the appearance of dark circles), which Ms. McCartney believes surpass the performance of many conventional — or potentially controversial — ingredients.

What’s so complicated?

Many consumers and brands believe natural ingredients are always better than lab-grown ones, but lab-grown ingredients can be less water- and labor-intensive. And “natural” doesn’t necessarily mean safer, given how many chemicals have been proven to be safe for use on skin.

Some ingredients that are popular in “clean” products like argan, juniper and shea are being overharvested, according to a report published last year by the Food and Agriculture Organization of the United Nations. Ingredients like sandalwood, for example, can be sourced from nature but can also be made synthetically, and companies that do so say a major incentive is to protect the environment.

And many naturally derived ingredients don’t undergo the same safety testing as synthetic or engineered ingredients, so they can cause irritation and allergy. Some studies have shown an uptick in skin reactions to essential oils, for example.

For anyone looking to better understand the ingredients listed on beauty and skin-care products, Mr. Talati of Goop pointed out that there are databases, like Skin Deep from the Environmental Working Group. There, consumers can find hazard rankings for controversial substances like lilial (recently banned by the E.U.) and parabens (often used as a preservative in cosmetics) and more run-of-the-mill substances like beeswax, which some dispute as clean because it is sourced from insects.

So is clean beauty here to stay or just another beauty trend?

Marcia Kilgore, the founder of Beauty Pie, a skin-care and beauty subscription service, noted the challenges for beauty businesses of all sizes in navigating the clean beauty era.

“If you don’t put ‘clean’ on product labeling, people assume there’s something wrong with it, but if you do, people say it’s a scam,” she said. She doesn’t think her customers necessarily require clean credentials, but they do want products that are safe and produce good results, be it from nature or a lab.

“To be clean is now just table stakes,” said Ms. Kilgore, a veteran of the industry. “The only way to gain attention in beauty is to claim something new. Soon it will be eclipsed by the next big thing.”

Still, scores of indie brands are putting “clean” values front and center, albeit alongside technological innovation — and a bit of dirt in the process. Haeckels, based in the British seaside town of Margate, has built a devoted following from innovations that include biocontributing mycelium packaging, prebiotic face masks and an odor-eating mushroom and kelp deodorant. At a time when the environmental value of refillable bottles in beauty is being questioned, Haeckels’ new vivomer packaging is compostable and made from microbes that are abundant in soil and marine environments.

Luonkos, a start-up from Finland, offers products with forest microbe extracts, as well as nettles, pine bark and birch bark powder.

Ms. Harper, who sold Tata Harper to Amorepacific Group, the Korean beauty giant, in September, believes that most consumers are now far more educated and curious about wellness regimens and the fact that clean products can be as effective as previous formulations.

“It is no longer a trend but rather the way the industry as a whole is headed,” she said. “That’s a necessary step to reducing the detrimental impacts of the beauty industry on the environment.”