These two terms may seem almost the same at first sight. However, if you are using OptiMonk, they mean different things.
You can meet the term „Impressions” in the Campaigns menu. It refers to the number of how many times your popup/popups has/have been displayed to your visitors.
Pageview is when a user visits one of the pages on your website. Pageviews are tallied for each page of your site.
For example, you have a visitor coming to your page. He will see the popup campaign once, so he will generate one impression for the campaign. But he does not stop at the home page, he continues browsing in the Categories page and 3-4 of the product pages as well. This way, he generates 5-6 pageviews. Usually, the pageview is a higher number than impressions, because popup campaigns often do not show for every visitor.