Source / Medium

Source/medium is GA4's combined dimension that names where a visit came from and how it arrived. The source is the origin — google, linkedin.com, a newsletter name. The medium is the type — organic, referral, cpc, email. Together, as google / organic, they're the granular layer underneath the tidier channel grouping.

Why it matters

Channels tell you the headline; source/medium tells you the story. "Organic Search is up" is a channel fact. "bing / organic doubled while google / organic held flat" is something you can act on. When you need to know exactly what drove a change — not just which bucket it landed in — source/medium is where the answer lives.

A concrete example

Your referral channel jumps 30% in a week. You open the source/medium dimension and see a single new row: news-site.com / referral with 220 sessions. That pins the spike to one mention on one site — far more useful than knowing only that "referral went up." You can now decide whether to nurture that relationship.

The common misreading

The mistake is misreading the special pairs. (direct) / (none) doesn't mean "people typed your URL" — it means GA4 couldn't determine a source, so it's the direct catch-all. And (not set) means missing data, not a real origin. Treat these as gaps to investigate, not as channels that earned their traffic. Untagged links are the usual culprit — fix them with UTM parameters.