- The definition of harmful is something that causes damage or is able to be hurtful.
A chemical that causes you to become sick is an example of a chemical that would be described as harmful.
(comparative harmfuller or more harmful, superlative harmfullest or most harmful)
- Nouns to which "harmful" is often applied: effect, consequence, impact, influence, emission, chemical, ingredient, substance, gas, agent, additive, drug, radiation, dust, organism, plant, animal, insect, action, act, behavior, component, content, activity, interference, use.
From harm + -ful.