A policy or set of guidelines setting standards and governing acceptable use of social media in the workplace, whether for personal use or marketing. 

Why do you need a Social Media Policy?

Policies help set expectations and can be used as a training tool, reference guide and resource for dealing with social media situations. For example, a complete social media policy would include the procedure employees should follow when facing a variety of social scenarios, such as a customer complaint or a request for information.

A number of examples can be found in this list of social media policies from companies large and small.