session
Also called: visit, sessions
In analytics, a session is a group of a single user's interactions with your site inside one time window. In Google Analytics 4 it opens with a session_start event and closes after 30 minutes of inactivity by default, so one visit can cover many pages and events.
How GA4 defines a session
GA4 fires a session_start event the first time a user opens your site with no active session. That event stamps a ga_session_id (the start timestamp in seconds) and a ga_session_number onto every event that follows, so a session is really a bundle of events, not a bundle of pageviews. It closes after 30 minutes of inactivity, adjustable up to 7 hours 55 minutes.
Two behaviors trip people up. GA4 does not reset a session at midnight, and it does not split a session when the traffic source changes mid-visit. Universal Analytics did both, so old benchmarks will not line up.
Counting is approximate. Since October 2021 GA4 uses HyperLogLog++ to estimate the number of unique session IDs, so a standard report can differ slightly from an exact BigQuery count. The trend stays reliable even when the raw number wobbles.
Watch the related “engaged session,” which needs longer than 10 seconds, a key event, or 2 or more page or screen views. For SEO you usually read sessions sliced by channel (Organic Search) and by landing page. Example: if organic sessions on one URL fall while its impressions in Search Console hold flat, the problem is more likely a CTR or intent mismatch than a ranking loss.
How it affects your traffic
Sessions are the unit you judge SEO performance by, so a slide in organic sessions is usually the first symptom you notice. The cause is rarely one thing: a de-indexed template, a slow page shedding users before the session engages, a redirect that drops the referrer, or tracking that double-counts can each show up the same way in a chart. An SEO Audit reads your session trends by landing page and channel, then traces each dip back to the technical or content cause you can actually fix.
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