Dimensions refer to attributes of your data that you use to categorise or segment your reports in order to answer business questions.

For example, 'date' is one of the most commonly used dimensions and would allow you to analyse your performance over time. Similarly, you may categorise your data by country and want to split out your performance by market - to do this you would want a 'Market' dimension. 

In the report builder, you can add in dimensions to your reports to segment your metric by dimension values. In the example below, my report would show impressions split by the dimension 'Market' and my dimension values would refer to specific markets e.g. United Kingdom, France, Spain etc.
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A dimension can be selected in the report builder by searching in the text box:

Or by clicking on the browse button under Dimensions section which will bring up a list of all the compatible dimensions in the report: