Content that is generally the same or is an exact match to other copy that is already published on the web. Put simply, Google doesn’t like indexing the same content more than once, as it’s a waste of resource on their side to crawl and index pages that contain the same copy. Nobody likes duplicate content and I think we can all agree that we look down upon those that deliberately steal content from others sites.
As an example, a lot of eCommerce sites have duplicate content, as they copy the manufacturer’s description of the product on their website – therefore, this content not only exists on their site but the manufacturers too. This is considered duplication, as the content already exists on another site that Google has crawled and indexed.
It has been confirmed that you cannot receive a penalty by having duplicate content on your website, however, the page on your site containing the same content may not rank as high, or may not rank at all, as it’s being filtered out by the crawlers.