Dimensions are characteristics, attributes of data like events or sessions tracked by Web analytics. A dimension is a description of your data that usually contains words rather than numbers. It’s a set of information that is sent with an event, e.g URL where the event occurred, a user’s browser, goal Conversion, revenue, etc. For instance, using page views as a metric and URL as a dimension you can find out how many times a given URL was viewed.

List of Piwik PRO dimensions can be found here:
Dimensions and metrics