A set of values by which you can group your data.
Dimensions are categories of information. The values contained in those categories are typically names, descriptions or other characteristics of that data.
For example, if you are reporting on website traffic, your data likely contains dimensions like Browser, Landing page and Campaign. The values for these dimensions will contain things like Chrome, or /home, or Summer Sale. Dimensions for geographic data would include Continent, Country, Latitude, and Longitude. The values for these dimensions will contain things like North America, United States, 36.985891, and -121.965528. Dimensions also include unaggregated numbers, such as price.