What is Duplicate content?

Content that appears on multiple pages or sites in identical or near-identical form. Google identifies the original source and pushes other versions to supplemental results rather than issuing a direct penalty.

From Duplicate Content and Anchor Text - #seopodcast 138

Content that is less than 75% different from content on another page or site, which can prevent Google from ranking both pages and risks a Panda penalty.

From Things That You Should Stop Doing With Your Website - Internet Marketing Podcast 189

Identical or near-identical text appearing on multiple pages or sites, a common issue for e-commerce stores using unmodified manufacturer product descriptions.

From SEO Myths Debunked - #SEOpodcast 242

Pages or text that are identical or nearly identical across a site or across the web. Google excludes duplicated pages from its index rather than ranking them.

From Internet Marketing ROI - Unknown Secrets of SEO E-Webstyle Number 35

Content copied from another source. Google identifies the original owner and will not rank the copying site for that content, and the site owner may also face legal liability.

From Internet Marketing Podcast - #seopodcast 37

Pages with identical or near-identical text that Google must choose between when deciding which to show, though the episode notes this is less of a concern for location pages that serve different geographic searches.

From Google Algorithm Changes and How it Affects Your Page - #seopdocast 165

Content that is identical or nearly identical across multiple pages or sites, common on ecommerce sites that copy manufacturer product descriptions, images, and titles.

From SEO for E-Commerce Sites - #seopodcast 145

Text that appears on more than one URL, which Google can detect and use to demote or remove pages from search results, as happened to the hosts' own podcast page.

From Mobile Websites and Video for SEO - SEO Podcast - Number 99