- 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.
harmful definition by Webster's New World
causing or able to cause harm; hurtful
harmful definition by American Heritage Dictionary
Causing or capable of causing harm; injurious.
- harmˈful·ly adverb
- harmˈful·ness noun