Any outbound link that is artificial, deceptive or manipulative. These links are usually paid for, created by scraping or spamming or a result of a low-quality SEO service and can earn you a manual action by Google. One way you can combat unnatural links is to have a good content strategy and only include reputable backlinks while disavowing the ones that you cannot remove yourself and are not valuable or natural.
About the Author: Chad Klingensmith
Chad is a Sr. SEO Strategist for Rio SEO. He has over 10 years of experience in the digital marketing vertical. He enjoys researching and applying cutting edge marketing strategies to help clients achieve optimal online visibility.