Impression:
The number of time your Ad is viewed i.e. It can be either a text Ad or maybe a image Ad, In any condition it would be counted as an ‘impression’ and will be recorded accordingly.
Click:
If a visitor clicks on a link or an Ad on your page it doesn’t mean that it would be counted as an additional Visit, There are chances of the page not being loaded or maybe the Google or any other tracking code not being loaded. If that is the case than the click would be counted as a ‘click’ not a ‘Visit’.
Pageview:
A “pageview” is the number of times the link on your website is viewed but this is just according to Googles search journal that “pageviews” exist else the same conditions apply but if there is no Google analytics code is installed it would be counted as an “impression”.
Hit:
A ‘hit‘ is anything that might get a request returned in exchange on a page.
Visit:
Once the page of a website opens completely that would be counted as a ‘Visit’.