What is Source?
In web analytics, Source is the label for where a visitor came from—typically a domain or system that initiated a visit or Session (e.g., google, bing, facebook.com, newsletter). It’s most often paired with “medium” (e.g., google / organic, newsletter / email) to describe both origin and transport. Source powers acquisition reporting, Attribution, and ultimately connects traffic to Conversion and Conversion Rate.
How Source works
Analytics tools determine Source using:
- UTM tags:
utm_sourceexplicitly sets the value (e.g.,utm_source=newsletter). See UTM and Campaign. - HTTP referrers: The browser’s Referrer header reveals the linking site; if it matches a search engine list, Source becomes the engine (e.g.,
google) and medium becomes Organic Search. - No referrer: If neither UTM nor referrer is present, the visit is usually classified as Direct.
Most tools stitch Source to a user via cookies or IDs (e.g., Client ID) and keep the first-touch vs. last-touch value per Session and across Pageview hits, depending on their attribution logic.
Common Source values (examples)
| Source type | Example values |
|---|---|
| Search engine | google, bing, duckduckgo |
| Social network | facebook.com, twitter.com |
| Referral website | partner.example.com |
| Email system | newsletter, mailchimp |
| Paid ad platform | google, facebook, adroll |
| Direct (none) | (no referrer / no UTM) |
Tool specifics (GA4, Matomo, Plausible, Simple Analytics)
- GA4: Prefers UTM; otherwise derives from referrer and its search engine catalog. Source is assigned at session start; later UTMs start a new session.
- Matomo, Plausible, Simple Analytics: Similar precedence (UTM > referrer > direct), but naming and grouping may vary slightly. Always standardize your UTM naming to keep Source clean.
Why it matters
A clean Source dimension makes acquisition cost analysis, Attribution, and channel trend detection trustworthy. Get UTMs right, audit redirects that drop referrers, and whitelist self-domains to avoid self-referrals.