How is a 'visit' or 'session' defined in Lytics?

A new session is started after 30 minutes of inactivity.

For example, the Audience rule "Total Number of Visits" is a count of the key _sesstart which is picked up by the Lytics Javascript tag. It is defned by: Length of time a between requests necessary before starting a new session. 1800 seconds by default.

See the tag configuration documentation for more info.