The shortest useful definition
In plain English: Dimension: what kind? Metric: how many or how much?
Country, session source and page path are dimensions. Active users, sessions and views are metrics. A report groups a metric by one or more dimensions.
- Country → Active users
- Session source → Sessions
- Page path → Views
Primary and secondary dimensions
In plain English: A second dimension adds another descriptive breakdown.
A table might start with Session default channel group, then add Device category. This can answer whether channel performance differs across desktop and mobile, but extra breakdowns can make sparse data harder to interpret.
Custom definitions
In plain English: Custom dimensions and metrics expose collected values GA4 does not already report.
Sending a custom parameter does not automatically make it broadly reportable. Register the appropriate custom definition, choose the correct scope and allow processing time before relying on it.
Compatibility and scope
In plain English: Not every dimension and metric represents data at the same scope.
User-, session-, event- and item-scoped fields can be incompatible or misleading when combined. Use Google's compatibility guidance rather than forcing every field into one table.
Keep in mind: When two reports disagree, check scope and filters before assuming collection is broken.