Analysis
Dimension
Also called attribute.
Dimension is a descriptive attribute used to split a metric into rows, such as page, country, or device. It is a label, not a number you add up.
How it is measured
Each hit or session carries values for several dimensions. A report groups by one, and the metric is summed inside each group. Cardinality, the number of distinct values, controls how readable the table is.
Check the dimension for stability. A page path with query strings can have thousands of values for one article. Clean values before you trust a ranking.
Worked example
A craft-supplies blog groups sessions by landing page. The result has 2,310 rows because tracking parameters make /yarn-guide appear 90 different ways. After stripping the parameters, the blog sees 312 real pages.
The top entry was /yarn-guide at 4,180 sessions once merged, not the 60 sessions each variant seemed to have. Nothing about traffic changed; the attribute was fixed.
How it differs
A dimension describes and slices. A metric measures and sums. 'Sessions by country' uses country as the dimension and sessions as the metric; swapping them makes no sense.
Common errors
Trying to sum a dimension. Using a high-cardinality dimension for a chart. Mixing page title and page path. Leaving case differences in UTM values. Treating 'not set' as a place.
In practice
Pick the three dimensions your team uses most and clean their values this week: normalize case, strip tracking parameters, and merge known aliases. Document each in one line.