“Clean” is not a regulated term in cosmetics. No agency defines it, no standard governs it, and no test qualifies a product for it. In practice each retailer publishes its own exclusion list, and those lists disagree with each other.
What it usually signals
Most clean ranges exclude a familiar set: certain parabens, formaldehyde releasers, some sulphates, synthetic fragrance, and a handful of others. If you react to something on that list, the label is a useful shortcut for you specifically.
Natural is not a safety rating
Poison ivy is natural. Several of the most common contact allergens in cosmetics are essential oils. Conversely, many synthetic ingredients exist precisely because they are more stable, better tolerated, and less allergenic than the botanical alternative they replaced.
Preservatives earn their place
Any product containing water can grow bacteria and mould. Preservative-free is not a virtue in a jar you keep in a warm bathroom for six months; it is a risk. Well-preserved products are safer products.
Read the list, not the badge
If you have a specific ingredient to avoid — an allergen, a sensitiser, something your dermatologist flagged — look for that ingredient by name. That is a real criterion. “Clean” is a category someone else defined, and they may not have defined it the way you would.