Negative keyword: Definition
A type of keyword that prevents your ad from being triggered by a certain word or phrase. Your ads aren’t shown to anyone who is searching for that phrase. This is also known as a negative match.
For example, when you add "free" as a negative keyword to your campaign or ad group, you tell AdWords not to show your ad for any search containing the term "free." On the Display Network, your ad is less likely to appear on a site when your negative keywords match the site's content.