Dimension

A dimension is a descriptive attribute you group data by.

Dimensions are the qualitative side of a report: country, device, page path, source. They answer "which" and "where," and you use them as the rows you break your numbers across.

In a report, "Country" is a dimension. Put it in the rows and GA4 splits your traffic into one line per country, so you can see where it comes from.

Dimensions describe, they never count. If you are trying to measure how much of something happened, you want a metric, not a dimension. Mixing the two up is the most common reason a report looks wrong.

Dimensions are the labels. Something else does the counting.