Authority can refer to a web page or to an author.

Page authority

Page Authority is a predictor the likelihood of a single page to rank well in search engines, irrespective of the content on the page. The higher this predictor, the more potential for that page to rank well in search results.

SEO Moz offers a proprietary page authority metric which is a score out of 100 based on a logarithmic scale.

Author authority

Author authority is a measure of the amount of trust that people can place in the content from a particular author. This authority is likely to be related to the following factors:

• Novelty: How novel the author's work is;
• Reliability: How reliable the information from the author is;
• Clarity: How well the author writes (grammar, spelling etc.);
• Consistency: Whether the message remains essentially the same for a topic;
• Engagement: How well the author engages with the expert or social group in the area of expertise.

In order to build authority on the personal level, users need to have a web presence in the form of a Blog and/or a social media presence. The authority of the user then will be assessed by search engines according to the number of likes, follows, shares, +1's etc. Developing authority takes a lot of effort, but this allows the author to stand out and is valuable for search engine ranking and web marketing.