By Google's own definition a "Hit" is any user interaction which results in data being sent to google analytics servers.  The most common of these hit types include page tracking hits, event tracking hits, and eCommerce hits.

Website owners use Google Analytics tracking code (embedded on their page) to trigger specific user behavior (better known as events) which are recorded through Google Analytics servers of said activity (for example, when a user loads the page of a website, it's considered a pageview event).  Also, beyond tracking just websites, hit events can be configured to track from mobile apps as well (for example, a screenview hit).

In summary, the most common hit types include, but not limited to:

• Page tracking hits (e.g. pageview)
• Event tracking hits (e.g. link clicks, page scrolling)
• eCommerce tracking hits (e.g. transaction and item data capturing)
• Social interaction hits (e.g. Facebook like button click)

More information from Google Support: https://support.google.com/analytics/answer/6086082

For specific details on various hit types and how they are used in Google Analytics tracking code: https://developers.google.com/analytics/devguides/collection/analyticsjs/field-reference#hitType