If a visitor enters to your website or application for the first time, or if they visit a page (or take any action that is tracked in Matomo) more than 30 minutes after his last action Matomo will record it in a new Visit.
A Unique Visitor that returns several times in one day can be recorded as multiple visits.
In their first visit to your website or application, the visitor is considered a New Visitor. From their second visit (possibly on the same day or even days or weeks later) they will be tracked as a Returning Visitor.
By default any action taken by a visitor within 30 minutes of their last action will be tracked to the same visit, this 30 minute default timeout can be changed by adjust the following in your config file:
[Tracker] visit_standard_length = 1800
Actions tracked in a new visit will not change any of the data tracked in the previous visit. For example, if a customer adds an item to their cart but does not complete the order within the same visit then that visit will always have an abandoned cart. Even if the visitor returns at a later time (In a new visit) and completes the order, the previously tracked abandoned cart will remain in the tracked data and reports.
See also our glossary of analytics terms.