From GA Support - A pageview is defined as a view of a page on your site that is being tracked by the Analytics tracking code. If a visitor hits reload after reaching the page, this will be counted as an additional pageview. If a user navigates to a different page and then returns to the original page, a second pageview will be recorded as well.
A unique pageview aggregates pageviews that are generated by the same user during the same session. A unique pageview represents the number of sessions during which that page was viewed one or more times.