One visit to your website. It ends after thirty minutes of inactivity, at midnight, or when the visitor arrives again from a different campaign.
Sessions are the unit most reporting is built on, so it is worth knowing what does and does not start a new one. The same person visiting three times in a week is three sessions and one user.
The thirty-minute rule produces effects that surprise people. Somebody who opens your site, takes a phone call, and comes back forty minutes later counts as two visits, which quietly inflates session counts on pages people leave open.
This comes up most often in Analytics & Conversion Tracking. Analytics, conversion events, call tracking and reporting installed and verified, so every enquiry can be traced back to what produced it.
A free consult is twenty minutes and there is no pitch deck at the end of it.