Deleterious means harmful or damaging, especially in a gradual or subtle way. When something has a deleterious effect, it causes damage that might not be obvious at first but becomes clear over time.

A lack of sleep has deleterious effects on your ability to concentrate and remember things. Constant negativity can have a deleterious impact on a friendship, slowly wearing it down until it falls apart. Scientists might study the deleterious effects of pollution on a river ecosystem, watching how contamination gradually harms fish and plant life.

The word suggests harm that accumulates or worsens with time, like rust slowly weakening metal or constant criticism gradually undermining someone's confidence. Deleterious is more formal than simply saying “bad” or “harmful.” You might hear it in science class when discussing how certain chemicals affect living things, or in a book describing how poor leadership had deleterious consequences for an organization.

The opposite would be beneficial or salutary (helpful). When doctors warn that something is deleterious to your health, they mean it causes real damage, not just a minor inconvenience.