A visit is also know by the term "session". It is one uninterrupted period of browsing a website. Salesforce defines the end of visit by inactivity for 20 minutes.
A visit can consist of one or multiple page views, across 20 minutes up to a virtually unlimited amount of time. Once the user does not request a new page view for 20 minutes, we consider the visit ended. A page view request 21 minute after the previous would be considered a second visit.