What is a “dimension” in Google Analytics?
A comparison of data between two date ranges.
A report that offers information about your audience.
An attribute of a data set that can be organized for better analysis.
The total amount of revenue a business has made in a given date range.
Dimensions are attributes of your data. For example, the dimension City indicates the city, for example, “Paris” or “New York”, from which a session originates. The dimension Page indicates the URL of a page that is viewed.
Read more here: https://support.google.com/analytics/answer/1033861