The officially stated definitions for LSI metrics can be found on the link below.
For visits it states the following;
"A visit is one or more page views over time by a uniquely identified client (visitor).
A cookie set in the browser identifies visitors (anonymous or registered), so if a visitor has cookies disabled, each page view counts as a new visit. The visit counter increments as soon as the visitor views any community page; the visit ends after 30 minutes of inactivity. After the 30-minute timeout, the next page view counts as a new visit. The Mobile Visits metric is a subset of the Visits metric and only includes requests from mobile web browsers."
Hope this helps.